Auto-save saves your latest editor text (into your account in the system database) every minute. If your browser crashes or you have a power cut etc. etc. the next time you start the editor (i.e. you restart a new post/reply) you will automatically get a 'Restore Auto Save' button at the bottom of the editor allowing you to restore the old text if you want to. It only saves the last text you were working on, so is overwritten every time you start a new post / reply.