As you drag in the timeline, it’s easy to place items imprecisely. To make dragging more accurate, use snapping. When the Snap feature is turned on, items will snap directly to the playhead, an edit point, markers, keyframes, or In and Out points. This makes it easier to align clips.
Look for a small gray arrow that appears below the point that indicates the item has snapped into place. If snapping is getting in the way of your edit or composite, however, turn it off.
The easiest way to quickly snap is with the keyboard shortcut S (as in snap). You’ll also see a magnet icon in the upper left corner of the Timeline panel.