In other words, a thread can have multipile locks in its hand. THese multiple locks may be obtainted by acquiring the same object's lock more than once OR obtaining one lock per object and more than 1 objects at the same time.

For example

case 1: lock_on_this and lock_onthis and lock_on_this

synchronized void m1 () {
   synchronized(this) {
      synchronized(this) {
          }    
   }
}

case 2 : lock_on_this & lock on objectRef1 and lock_onObjectref2 

synchronized void m1 () {
   synchronized(objectRef1) {
      synchronized(objectRef2) {
          }    
   }
}