The word "monitor" in Java refers to obtaining LOCK on an object. If you know a little about threads in java. This concept of monitor is related to threads. Every object has its own lock or you can say every object has its own monitor. This lock comes into a major use when we use synchronized methods/block. Once a thread enters a synchronized method/block it automatically gets a lock on the current object or instance of class. And how does this help? Obtaining lock will ensure that no other thread will be able to use that object until this thread releases the lock. So any modification done to the state of Object can be done only by the current thread who has the lock and no other thread can simultaneously make changes to the state of the object. I hope this clears it.
But you will understand it better after you read SYNCRONIZED CODE section of K&B book.