"RTFM" is an acronym for "Read the Fine1 Manual". It is typically used in response to a frequently-asked question ("FAQ") to mean "Your question is answered better and in more detail by the manual than I could tell you now, so please look there." If you are told this, usually the FAQ (Frequently Asked Questions list) is a good place to look for the answer.
1 Some people substitute an alternative word here. If you don't know or can't guess what it is, you're better off not knowing.