In the menu Windows > Others, you will find a History panel. The contain of the History panel depends on the project.
Latest actions are at the top of the list. The oldest are at the bottom. Same actions are memorized in groups. For example, if you draw with a brush and then, change your tool, strokes made with the first brush will be put into a group.
The history panel allows you to save the creation's steps. For example, if you hesitate between two renders for an illustration, you can save the both compositions and navigate from one to another.
To save an action, click on the action group that interests you and mark an action by clicking on the empty square or the icon : it will now contain a little cross.
If you mark several actions, you will have the possibility to navigate from a mark to another one thanks to the arrows icons : and .