what does it mean to preempt a tv show

To preempt a TV show means to replace or take the place of a regularly scheduled program with another broadcast that is considered more important or urgent. This can happen, for example, to air special priority content such as a presidential speech, breaking news, a live sports event that runs longer than planned, or emergency coverage. When a show is preempted, it is either delayed, rescheduled, or sometimes canceled for that time slot to make way for the more important broadcast.