An Acronym. You want to know what it means? rtfm(7) already!

See also: rtfm(1), STFW

ESR's How To Ask Questions The Smart Way explains:

Often, the person sending either of these replies has the manual or the web page with the information you need open, and is looking at it as he types. These replies mean that he thinks (a) the information you need is easy to find, and (b) you will learn more if you seek out the information than if you have it spoon-fed to you.

You shouldn't be offended by this; by hacker standards, he is showing you a rough kind of respect simply by not ignoring you. You should instead thank him for his grandmotherly kindness.