When you edit a category or article, autosave creates a backup every 30 seconds.
If you leave without saving your changes, autosave will let you know the next time you come back to the category or article. You'll see a Recover auto-save link just above the editor:
Click this link to open a pop-up where you can choose whether to delete or recover that autosave:
Once you've recovered the autosave, you'll still need to Save the article to permanently save those changes.