The Timeline represents individual frames of animation within a Timeline animation, or can represent differing sections of an ActionScript animation.

Timeline in Timeline based animation

In this format, the Timeline acts something like a VCR. It has individual frames to hold single images or tweens and a playhead that scrolls through them, making each still appear animated just as traditional animation does. Each individual square on the Timeline represents a frame and every fifth frame is labeled and darkened. The Timeline is also where layers are located and managed. While working with Flash, one can press enter at any time to have the playhead scroll forward and play the animation.

Timeline in ActionScript based animation

In an ActionScript animation there will be fewer frames used, and often a developer can get away with using just a single frame since their object's animations are created with code rather than a series of pre-existing images. Multiple frames are still sometimes used in order to make projects more organized.