A mutex is an exclusive lock that threads can own and disown, in order to synchronize between them. See Mutexes.
Mutex objects have no read syntax. They print in hash notation, giving the name of the mutex (if it has been given a name) or its address in core:
(make-mutex "my-mutex") ⇒ #<mutex my-mutex> (make-mutex) ⇒ #<mutex 01c7e4e0>