The Edit menu groups together the actions related to the edition, and so will be mostly used while in Editor mode.

Undo (action)Reverts the last editing operation, namely action
RedoDoes the last editing operation again, after an Undo
Undo Selection ChangeReverts the last selection operation
Redo Selection ChangeDoes the last selection operation again after an Undo Selection Change
CutDeletes the current selection, but puts it in memory ready to be pasted
CopyCopies the current selection to memory
PastePastes the memory at the edit point, after a Cut or Copy operation
Select
Select All ObjectsSelects all the regions and automation pints in the session
Select All TracksSelects all the tracks, busses and control masters in the session
Deselect AllDeselects all objects or tracks, nothing is selected
Invert SelectionSelect the previously unselected regions, and deselect the previously selected ones
Set Range to Loop RangeCreates a range selection on the selected tracks, based on the selected loop markers, and switches to Range Mode tool
Set Range to Punch RangeSame as above, based on the selected punch markers
Set Range to Selected RegionsSame as above, based on the selected regions (i.e. from the start of the earliest region to the end of the latest one)
Select All After Edit PointSelect all the regions and automation points that exist after the Edit Point, even if the region starts before it. If some tracks are selected, only selects on these tracks.
Select All Before Edit PointSame as above, but before the Edit point (i.e. to the left of it)
Select All Overlapping Edit RangeSelect all the regions and automation points of which at least a part is in the current selection range
Select All Inside Edit RangeSelects all the regions that are completely inside the selection range, i.e. their start and end are inside the range. If some tracks are selected, only selects on these tracks.
Select All in Punch RangeSelects all the regions of which a part in in the punch range. If some tracks are selected, only selects on these tracks.
Select All in Loop RangeSame as above, based on the loop range
Move Range Start to Previous Region BoundaryExtends the left boundary of the range to the left to the next region start or end. The region must be in the range.
Move Range Start to Next Region BoundarySame as above, to the right (reduces the selection)
Move Range End to Previous Region BoundarySame as above, with the right edge of the range, to the left (reduces the selection)
Move Range End to Next Region BoundarySame as above, with the right edge, to the right (extends the selection)
Start RangeSets the left edge of the range to the Edit point
Finish RangeSets the right edge of the range to the Edit point
Select Next Track or BusSelect the track or bus under the currently selected one. If multiple tracks are selected, only the first one is considered
Select Previous Track or BusSame as above, with the track/bus above the first one selected.
DeleteDeletes all that is currently selected
CropCuts the parts of the regions that are outside the range boundaries. Only applies on the regions that belong at least in part to the range.
Split/SeparateCuts the selected regions at the Edit point, separating them in two regions
Separate
Separate UnderRemoves all the parts of the regions that are under the selected one. Once done, the selected region is alone on its part of the track.
Separate Using Loop RangeCuts the selected regions or the regions on the selected tracks along the Loop range's start and end markers. If nothing is selected, acts on all tracks at once.
Separate Using Punch RangeSame as above, with the Punch range markers
Align
Align StartMoves the selected regions to align the beginning of the regions to the Edit point
Align Start RelativeWhen multiple regions are selected, moves all the regions together as a block to align the beginning of the earliest one to the Edit point.
Align EndMoves the selected regions to align the end of the regions to the Edit point
Align End RelativeWhen multiple regions are selected, moves all the regions together as a block to align the end of the latest one to the Edit point.
Align SyncMoves the selected regions to align the Sync point of the regions to the Edit point
Align Sync RelativeWhen multiple regions are selected, moves all the regions together as a block to align the earliest Sync point to the Edit point.
Fade
Fade Range SelectionFor all the regions that either begin or end in the range, create a fade in or out on the regions length.
Set Fade In LengthIf the edit point is withing the region boundaries, adjusts selected audio regions' fade in to end at the edit point.
[] Fade InToggles the fade in on the selected region on or off
Set Fade Out LengthSame as above, for the fade out
[] Fade OutToggles the fade out on the selected region on or off
Remove Last CaptureDestroy the last recording. A prompt reminds the user this cannot be undone.
Edit point
Change Edit PointToggles between the mouse and the playhead as the Edit point
Change Edit Point Including MarkerToggles between the mouse, the playhead and marker as the Edit point
Snap Mode
() No GridDisables snapping, i.e. allows free movement of regions and boundaries
() GridForces snapping, so any move of region boundary will be lined to the grid as chosen
() MagneticIf the movement of the region or boundary happens near a grid line, snaps, otherwise, allow free movement
Next Snap ModeToggles between the No Grid, Grid and Magnetic snap modes
Next Snap ChoiceCircles through the snap choices, as detailed below
Previous Snap ChoiceCircles through the snap choices, as detailed below, in reverse order
Next Musical Snap ChoiceCircles through the musical snap choices, e.g. those expressed in bars and beats
Previous Musical Snap ChoiceSame as previous, but in reverse order
Snap To
Snap to CD FrameThe grid unit will be 1/75th of a second
Snap to Timecode Frame/Second/MinuteThe grid unit will be based on the timecode settings for the session
Snap to Second/MinuteThe grid unit will be based on absolute times
Snap to nthThe grid unit will be 1/n beats and will depend on the tempo and meter in effect at that point in the timeline.
Snap to BeatSame as above, whole beat
Snap to BarSame as above, whole bar
Snap to MarkThe grid will be made of markers
Snap to Region StartNo grid, the regions will snap to the closest region start on any track
Snap to Region EndSame as above with the regions' ends
Snap to Regions SyncSame as above, with the Sync points (by default, start of the region)
Snap to Region BoundariesSame as above, for both the starts and ends of regions
Tempo
Set Tempo from Region = BarComputes the tempo so that the duration of the first selected region is 1 bar. Ardour prompts if the user wants it to be the global tempo, or a tempo marker at the beginning of the region used
Set Tempo from Edit Range = BarSame thing, with the current Range instead of a region
[] Smart Object ModeToggles the Smart Mode, allowing the mouse to be in Range Mode in the upper half of a region, and in Grab Mode in the lower half
Scripted Actions
[] Script ManagerShows the Script manager, allowing to use and manage the Lua scripts in the session
Unset #nDeactivate the nth script
PreferencesDisplays the Preferences panels, allowing to change Ardour's behaviour