The timeline will contain the keyframes, frames, programming, sound, and pretty much everything in your published Flash file.

When you first open a New Flash file, you will only have one scene and the file will have "Scene 1" written above the timeline. You can have many timelines in Flash: multiple scenes and symbol timelines. Pay attention to the type above the timeline to find out which timeline you are looking at.

The timeline has a playhead (red bar) that indicates the frame number you are currently looking at.

Hints: think of each frame as a new piece of paper.

To add Frames: F5 key
To delete Frames: Shift + F5 key

To add Keyframes: F6 key
To delete Keyframes: Shift + F6 key

To add Blank Keyframes: F7 key

You can also do this via the menu, but they may change in location from Flash version to version, so it is best to stick with the hot keys.

As of Flash MX 2004, these options are under:
Insert>Timeline for adding
Modify>Timeline for deleting