View Snap to Grid Icon
When the Snap to Grid button is enabled and a selection is made in an area of the mix, the left and right borders of the selection will automatically align themselves with the vertical grid lines. These grid lines represent audio down beats, or fractions thereof, depending on the viewing resolution. In general audio will sound best when cut on a grid line.
Also, when enabled and a beat synced audio segment is sized left or right, the segment boundary will align itself with the nearest grid line.
You can also disable this function by right-clicking in the Mixing Window and clicking the Snap to Grid option. You can now move the ends of the Segment / Selection Box freely without snapping to the nearest grid lines.
Watch Video: Snap to Grid Icon (Mixing Screen Video Part 1)