"RTFB" is short for "read the fucking binary", in a similar way as "RTFM" means "read the fucking manual" and "RTFS" means "read the fucking source". While both "RTFM" and "RTFS" have legitimate uses, "RTFB" is usually intended only as a parody of them. While any user, no matter how tech-savvy, can be expected to read the manual, and experienced computer programmers can deduce the working logic of a computer program directly from reading its human-readable source code, "RTFB" expects users to directly read the machine language code that the microprocessor executes natively. Today's microprocessors are usually so complex that, except for trivial textbook programs, this is a difficult task even for experienced programmers, and because of anti-reverse-engineering laws, may even be illegal.
A more recent use, applied to members of the legislative branch, means "read the fucking bill". It is a reaction to the practice of passing massive legislation where the final bill is available for only a few hours before it is voted upon.