A monitor is mechanism to control concurrent access to an object.
This allows you to do:
public void a()
// do something (1)
public void b()
// do something else (2)
This prevents Threads 1 and 2 accessing the monitored (synchronized) section at the same time. One will start, and monitor will prevent the other from accessing the region before the first one finishes.
It's not a special object. It's synchronization mechanism placed at class hierarchy root: java.lang.Object.
There are also wait and notify methods that will also use object's monitor to communication among different threads.