What is mean by interrupt service routine?

It is a routine in a micro controllers program where the program jumps to after receiving a interrupt on the micro's interrupt pin. Because most micro's can only run one programme at a time, one need a way for the micro to stop doing what it's busy with, to service an outside peripheral that need urgent attention. For instance the micro is running a flashing LED, a push button that one want to use to stop the flashing and start to flash another LED is connected to the interrupt pin of the micro then the program save what it was busy with on a stag and immediately start the interrupt routine.