The History Brush tool allows you to “paint” back portions of your image to an earlier state. Think of it as the clone tool, but the source is a previous version of your current image. By default the history brush tool will paint back the pixels found on your image when it was first opened.
If you want to change the default history brush state, open the History panel by going into Window > History. Then define which History state you will use, by clicking on the box to the left of any of the history states. Once you click on a box you will see the history brush icon inside that box, indicating that the history brush will reference that state while painting.