When you're working in the Timeline, you are determining when objects appear and disappear and how and when they are animated. You can control this timing by:
When you want to synchronize two or more objects - or their animations with each other, it might be interesting to switch on the 'Snap' option. Just click the Snap icon you find top left in the Timeline:
As of now, when you change the timing of an object, a Clip, or an animation by dragging one of the edges or keypoints, it will snap to the edge or timing of other objects or animations nearby. While dragging with your mouse, you will feel that the edge or keypoint you are dragging jumps to the timing of another edge or keypoint nearby, and takes over its exact timing:
Depending on the visual effect you have in mind, you may also want to switch off the snapping so that this snap behavior does not happen unintentionally.