An interrupt is a notification that a monitored event has occurred. In a program, it will be an event that is being listened for but isn't holding up the entire program's operation waiting for the event to happen. When the even does happen, this triggers an interruption to let the program know that the event has occurred. Using interrupts with the RPi.GPIO library is a powerful tool for receiving new information as it happens, rather than checking periodically to see whether new information exists or sitting and waiting on an event to occur.

A real-world analogy would be baking a cake. You put the cake in the oven, set a timer, and then go on to doing other things. You don't have to wait there by the oven for the cake to bake, because at some point, the timer will interrupt you to let you know you need to check on it. A program may log data to a file during normal operation but, if sent an interrupt, will know it has to do something different...