About the History panel
The History panel shows a list of the steps youve performed in the active document since you created or opened that document, up to a specified number of steps. (The History panel doesnt show steps youve performed in other frames, in other Document windows, or in the Site panel.) It allows you to undo one or more steps; it also allows you to replay steps and to create new commands to automate repetitive tasks.
The slider, or thumb, in the History panel initially points to the last step that you performed.