RFC, or Request for Comments, are documents published that describe various items surrounding computer networking. Generally, these are memorandums published by the Internet Engineering Task Force.
RFCs can be a great resource. For some unknown reason, most candidates preparing for the CCIE don't take the time to review these documents, which can be very helpful in assisting with understanding the how and why of various networking components. Perhaps the language is a bit dry, or they prefer books with shiny covers.