The Transport menu handles how Ardour handles the playback and playhead.

Start/StopStarts or stops the playhead, and recording if it's armed
Play
Play SelectionOnly plays the selected part of the session, be it a range or selected regions
Play Selection w/PrerollAs the previous menu, except it starts the playback 1/2 bar before the beginning of the selection
Start/Continue/StopFIXME
Play from Edit Point and ReturnStarts the playback at the Edit point, and when stopped, goes back to the orginial location
Play Loop RangeIf a Loop range is defined, play it and loop until stopped
Start RecordingThis is a shortcut to trigger the global recording, and start playback at once
Stop and Forget CaptureStops the recording, removes the newly created material, and goes back to the original position
Enable RecordTriggers the global recording. Next time "Play" is pressed, it will record on the track(s) that are armed for recording
Set Loop from SelectionConverts the selection into a Loop range by placing loop markers at the start and end of the selected range
Set Punch from SelectionSame thing, for Punch
Set Session Start/End from SelectionSame thing, for the start and end markers of the session, defining the sessions length
ForwardFIXME
RewindPlays the audio backwards from the playhead on
Transition to RollFIXME
Transition to ReverseFIXME
Playhead
Playhead to MouseSet the position of the playhead at the current position of the mouse cursor
Playhead to Active MarkIf a marker is selected, set the position of the playhead at the position of the marker
Center PlayheadCenters the view on the playhead without changing the zoom level (putting the playhead in the middle of the screen)
Nudge Playhead ForwardShifts the position of the playhead to the right by the amount shown in the nudge timer
Nudge Playhead BackwardSame thing, to the left
Move to Next TransientWhen transient have been set, moves the playhead to the next one to the right
Move to Previous TransientSame, to the left
Playhead to Next GridRegardless of the state of the Grid Mode, goes to the next grid to the right, as set by the Snap/Grid unit
Playhead to Previous GridSame, to the left
Playhead to Next Region BoundaryMoves the playhead to the right to the next beginning or end of region on the selected track or, if no track is selected, on all tracks
Playhead to Previous Region BoundarySame, to the left
Playhead to Next Region Boundary (No Track Selection)Moves the playhead to next beginning or end of region, be it on the selected track or any other
Playhead to Previous Region Boundary (No Track Selection)Same, to the left
Playhead to Next Region SyncMoves the playhead to next Region Sync Point, that is by default the beginning of a region but can be moved
Playhead to Previous Region SyncSame, to the left
Jump to Next Markmoves the playhead to the next marker on the Ruler
Jump to Previous MarkSame, to the left
Go to ZeroSends the playhead to the 00:00:00:00 time, regardless of the sessions Start marker
Go to StartSends the playhead to the Start marker of the session
Go to EndSends the playhead to the End marker of the session
Go to Wall ClockSends the playhead to the current value of system time, as shown on the top right of the Status bar
Active Mark
To Next Region BoundaryMoves the currently selected marker to the next region beginning or end
To Previous Region BoundarySame, to the left
To Next Region SyncMoves the currently selected to the next region sync point (by default: beginning or end of the region)
To Previous Region SyncSame, to the left
Markers
Add Mark from PlayheadCreates a Marker at the position of the playhead
Remove Mark at PlayheadRemoves any marker at the position of the playhead
Toggle Mark at PlayheadCombine the 2 previous: if a marker exists, deletes it, otherwise create it
Locate to Mark nIf it exists, goes to the n-th marker
Set Session Start from PlayheadPuts the Start of the session marker at the playhead's position
Set Session End from PlayheadPuts the End of the session marker at the playhead's position
[] Time MasterSets Ardour as the Time master, i.e. Ardour sends the time information to the audio system
[] Punch In/OutBased on the Punch in and Punch out markers if they exist, tells Ardour to record only between those two points
[] Punch InBased on the Punch in marker, only allow to record from this point on
[] Punch OutBased on the Punch out marker, forbids recording before this point
[] Audio InputIf checked, automatically switch the monitor from input to playbackmode when playing
[] Follow EditsIf checked, selecting a region moves the playhead to its beginning
[] Auto PlayIf checked, moving the playhead in the ruler starts the playback
[] Auto ReturnIf checked, when the playback is stopped, go back to the previous position of the playhead. If not, the playhead stays where it is when the playback is stopped
[] ClickActivates/desactivates the click track (metronome)
[] Follow PlayheadIf checked, while playing, when the playhead reaches the right of the screen, Ardour scrolls one screen to the right to keep the playhead visible at all times
[] Stationary PlayheadIf checked and if Follow playhead is checked, on playback, the playhead stays at the center of the screen, and the session scrolls
PanicImmediately stops all MIDI playback (usefull e.g. when a MIDI bug in encountered)