A thread in Emacs represents a separate thread of Emacs Lisp execution. It runs its own Lisp program, has its own current buffer, and can have subprocesses locked to it, i.e. subprocesses whose output only this thread can accept. See Threads.
Thread objects have no read syntax. They print in hash notation, giving the name of the thread (if it has been given a name) or its address in core:
(all-threads)
⇒ (#<thread 0176fc40>)