tfw is an acronym meaning "the fuck what" (usually followed by a question mark)
it's used to express being confused or amazed by something.
It's also used by Star Wars geeks to imitate the way Yoda would say 'what the fuck'
guy #1: Hey, have you heard about that guy that could bend all the way over until his head is under his ass and then shit on himself?
guy #2: tfw!?