Old joke explained: RTFA means Read The Fine Article -- except that F doesn't really stand for fine. It's usually meant as a crabby way of saying "The article answers your question".

From that, "TFA" just means "The Article", but without any of the crabbiness, and without the F really standing for anything. That's just a shortcut, and a nice instance where a bit of Internet lore became nicer rather than meaner.