The timeline is one of the most important toolbars in Flash. On a timeline you have frames and keyframes. A frame is simply an intermediary between keyframes, there is no change in the amount of objects on the stage during a normal frame. Keyframes are major changes in the state of the stage. You use them to add a new object or end/begin an animation.
Keyframes are marked by small squares with a black dot in them. Blank keyframes are marked by a square with an empty dot. You can insert a frame with F5, a blank keyframe with F7, and a non-blank keyframe with F6. (You can also insert frames and keyframes from the contextual menu, which can be accessed by right-clicking on the timeline.)
All commands to insert and delete keyframes are available by right clicking (Mac: Control Click) on any frame of the timeline.
On a timeline there are layers. A layer can contain only 1 animation at a time, so therefore to animate multiple objects you need to use multiple layers to do multiple animations. Any layer above another layer will have the layers contents stacked above the lower layer.
Here we have the upper portion of the interface in Flash, the timeline window. This area is used to edit the work file frame by frame. Key frames, motion tweens, onion skins are some of the things that can be added/edited here. The left portion of this window also allows for the option of several layers of frames.