Timeline Animations are used to control the "In" and "Out" presentation of Subcompositions, Groups, and Widgets within a Composition Tree over time.
If "2 timelines" is turned off, the "Out" animation will be a reverse of the "In". The Timeline is incremented in seconds, and includes features such as zoom, snap to grid.
Timeline Animation Tab (named "Animation") in the Output Window
The Timeline Animation is one of three methods to animate within Composer. The other two are Behavior Animations and Update Animations.