This is the timeline:
It shows the layers and frames of the active sprite. You can use the
View > Timeline menu option or press the
Tab key to show or hide
it. It also appears automatically when you create a new Frame or
Layer. (This behavior can be disable from Edit > Options > Timeline >
Show timeline automatically.)
You can use Layer > New Layer menu (
Shift+N) to create new layers
or Frame > New Frame menu (
Alt+N) to add frames.
You can configure the Timeline pressing the button.
You can copy and paste layers, frames, or cels between different documents using Edit > Copy and Edit > Paste.