The specific definition can be somewhat fluid, buy basically you are being made to fly when your schedule said you were to be off. This can cause HUGE problems for people that commute or have families to attend to. Control over one's schedule is the only thing that makes this life style palatable. When other people's lack of planning (scheduling/planning) infringes upon that, you pay the price. The term comes from the fact that these surprise assignments are usually supposed to got to the most junior pilot they can snag. not always the case.