It is the timeout, say you walk up to your printer and make 99 copies of something.
Later another user goes to make 1 copy, so they load their original and hit the start button. Without auto clear they would get 99 copies.
This also applies to Accounting, if a user logs in and makes a copy, with no auto clear, everyone in the building would for the rest of the day be making copies that would get billed to the first person who forgot to manually log out.
To change the timer settings from 30-900 seconds, or disable it go as admin to Machine Status > Tools > System Settings > Machine Clock / Timers > Auto Clear