The timeline is a matrix of frames and layers. Think of it like a spreadsheet where A1 is the first cell. The letter refers to the column and the number refers to the row. In Flash it is analogous, but the columns are called frames and the rows are called layers. The Frame numbers appear across the top of the timeline and the layer names go down the left column. The animation plays from the beginning to the end based on placement of objects along the timeline. Code, called ActionScript, can alter that timeline; this would be the trademark of a more complex animation. We will focus on a basic left-to-right timeline animation.
The red playhead selects which part of the timeline is visible on the stage. Open BasicAnimation/demos/FrameByFrame.fla and drag the playhead back and forth. This is called scrubbing the timeline.
Keyframes are the only frames you may edit. All Keyframes have small circles on them. Keyframes with visible content have solid circles while blank keyframes have hollow circles.
Click on the several different keyframes and watch the content on the stage change. You can hide or lock any layer by clicking on the dot under the eyeball (to hide) or the lock (to lock). Click on the eyeball or lock directly to hide or lock all layers at once.
In Photoshop hidden layers are not part of an exported file, but in Flash they are. Even hidden layers will be included in the exported document (unless you change this in the Publish Settings).