Ardour's editor utilizes a grid to assist in the placement of regions on the timeline, or with editing functions that need to happen at a specific point in time. You can choose if you want the cursor and various objects to snap to this grid, and how you want the snapping to behave. You can modify the grid units to fit your needs.

About Snapping

There are two ways to think about aligning material to a grid. The first and most obvious one is where an object's position is clamped to grid lines. In Ardour, this is called absolute snap and is commonly used when working with sampled material where audio begins exactly at the beginning of a file, note or region.
The second, relative snap, is used when an object's position relative to the grid lines is important. In music, this allows you to move objects around without changing the "feel" (or timing) of a performance.
Absolute snap is the default method of snapping in Ardour.
While dragging objects you may switch from absolute to relative snap by pressing the absolute snap modifier key(s).
You may also disable snap entirely by using the snap modifier (see below).
Note that in relative snap mode the reference point is taken to be the distance to the nearest grid line.
Note also that when an object lies exactly on a grid line, there will be no difference between relative and absolute snap modes.
The realtive snap and snap modifiers (along with other modifier keys) may be set in Edit > Preferences > User Interaction
For common use patterns, it is recommended that you assign a unique key for one snap modifier and two keys for the other in such a way that they share an otherwise unused key. For example, you may choose the snap modifier to be the key and the relative snap modifier to be the and keys.

Snap Modes

Using the above modifications, Ardour supports three different modes of snapping to the grid:

No Grid: disables the grid. All objects move freely in this mode.
In No Grid mode, you may temporarily activate the grid by pressing the snap modifier (for absolute snap) or switch to relative snap by pressing the relative snap modifier.
Grid: activates normal snapping. All positions of objects snap to the grid. (See Grid Units below to change the grid). If you try to move an object in "Grid"-mode, it does not change its position until you move the mouse far enough for the object to reach the next grid line.
Sometimes you may wish to maintain an objects' position relative to the grid line. In order to do this, use the "snap relative" modifier. When holding down this modifier during a drag, the dragged object will jump while maintaining its original distance from the line.
New objects will always be created at grid points.
Holding down the snap modifier will disable the current grid setting and allow you to move the object freely.
Magnetic: is a less strict type of snapping. Objects can still be moved to any position, but positions close to the relative or absolute grid points will snap. In order to move an object very close to a snap point, it may be necessary to zoom in to prevent snapping to that point, or to use the snap modifier to disable snap completely.
As with Grid mode, the snap modifier will disable snap completely while the absolute snap modifier will move the "notch" of Magnetic snap to the grid lines.

Syncing Regions to the Grid

By default, a region's beginning will be used as the reference for both types of snapping, but you can change this behaviour by setting a sync point in the region. Select the region(s) and press V. This will set the sync point to your edit point.

Grid Units

The selector next to the grid mode selector defines the size of the grid elements. You can set your grid to several different units:

CD Frames: A CD Frame is 1/75th of a second. Snapping to CD Frames (using absolute snap) can be used to avoid issues with CD track lengths.
Timecode Frames/Seconds/Minutes: The duration of a frame depends on the timecode settings for the session.
Seconds/Minutes: These are absolute time units, unaffected by sample rate or timecode settings
Beats/N: Set the grid to units of 1/N beats, where N can be 128, 64, 32, 16, 8, 7, 6, 5, 4, 3, 2. The duration of a grid unit will depend on the tempo and meter in effect at that point in the timeline.
Beats: Set the grid to whole beats. The duration of a grid unit will depend on the tempo and meter in effect at that point in the timeline.
Bars: Set the grid to whole bars. The duration of a grid unit will depend on the tempo and meter in effect at that point in the timeline.
Markers: The grid lines are the markers.
Region Starts: The grid lines are constructed from region start points (see below).
Region Ends: The grid lines are constructed from region end points (see below).
Region Syncs: The grid lines are constructed from region sync points.
Region Bounds: The grid lines are constructed from region start or end points.

To use Region starts/ends/syncs/bounds as snap choices, you must have either

No tracks selected, which means that Ardour snaps to regions on any track, or
Several tracks selected, which means that Ardour only snaps to regions on those selected tracks.

If you are moving items on a track, and only the current track is selected, then you will only be able to snap to other regions on the same track. This means that enabling Edit > Preferences > Editor > Link Selections of Regions and Tracks will make the "Region" grid unit unusable. Avoid the use of this option if you are going to use any of the Region grid units.