A Rollback is the reversion of in-game content to an earlier state. Rollbacks within RuneScape are used under very strict and rare circumstances. They are generally used to eradicate a problem that cannot be corrected by conventional methods. A rollback can also be referring to a Jagex mod resetting a player's stats.
Cases of Rollbacks Edit
There was a rollback on Feb 6th 2016. People reported cases of being rolled back at least 20minutes. This was most likely due to a bug where players were finding themselves unable to log in for hours; these players got the "your account is already logged in" message even though their accounts weren't logged in.