The History Palette provides a history of all the actions that you have made in the timeline and Controls window. To open the History Palette, choose Window > Show History Palette.
You can use this window to review previous actions or to undo back to a specific change by clicking that change in the History Palette. Once you click the appropriate action in the window all the actions up to and including the selected action are undone.
Events that have been undone are greyed out, but still remain in the History Palette until new actions are taken. If you would like to redo a removed action, simply click on the greyed out option to reactivate it.