This is the timeline:
It shows the layers and frames of the active sprite. You can press the
Tab key to show or hide it. Also it appears automatically when you
create a new Frame or Layer. (This behavior can be disable
from Edit > Options > General > 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.
See the Onion Skinning section.