If a player plays at the same limit for an extended period of time, even though he has the money to move a limit up, he plays overrolled. The term is derived from bankroll. Usually you play overrolled because you do not want to be scared money to the tables after the ascent. Another reason may be that you do not consider yourself strong enough yet and prefer to gain more experience at the lower limit.
One advantage of this is that you can approach a limit climb more relaxed, because a small downswing does not mean the re-descent.