The difference is simple: if the locked-on object is in a static field, then all instances of MyClass*will share that lock (i.e. no two objects will be able to lock on that object at the same time).
If the field is non-static, then each instance will have its own lock, so only calls of the method on the same object will lock each other.
When you use a static lock object:
thread 1 calls o1.foo()
thread 2 calls o1.foo(), will have to wait for thread 1 to finish
thread 3 calls o2.foo(), will also have to wait for thread 1 (and probably 2) to finish
When you use a non-static lock object:
thread 1 calls o1.foo()
thread 2 calls o1.foo(), will have to wait for thread 1 to finish
thread 3 calls o2.foo(), it...
