This is a threading question and is probably better in the Threads forum. I'm sure a bartender will move it there for us.
Mutex is short for Mutual Exclusion. You can read more about them in the Mutual Exclusion wikipedia article. To the best of my knowledge, Java does not have Mutexes directly, but it does have Intrinsic Locks via the synchronized keyword, which act like Mutexes. Java also has Semaphores which are similar to Mutexes. The Semaphore Wikipedia article discusses the differences between a Mutex and a Sephamore.