If any thread enters into a synchronized block.Then that thread is having the lock on that object.If any thread wants to enter into that synchronized block,they have to wait untill the thread holding the lock completes it's execution(Means leaving the lock)
Its pretty much clear what anil has said ........i dont think an example is needed to make it understand.........!A thread can hold multiple objects lock meaning to say instances of many classes ..........!Gotcha ?