A REENTRANT LOCK IS ONE WHERE A PROCESS CAN CLAIM THE LOCK MULTIPLE TIMES WITHOUT BLOCKING ON ITSELF. IT'S USEFUL IN SITUATIONS WHERE IT'S NOT EASY TO KEEP TRACK OF WHETHER YOU'VE ALREADY GRABBED A LOCK. IF A LOCK IS NON RE-ENTRANT YOU COULD GRAB THE LOCK, THEN BLOCK WHEN YOU GO TO GRAB IT AGAIN, EFFECTIVELY DEADLOCKING YOUR OWN PROCESS.
explains clearly