DFAC is military lingo for a cafeteria, but here it's more than just a cafeteria, it's an only source of food for places without much by way of a commisary or PX.
Though of significantly lower quality than any given civilian establishment, it's guaranteed to be open 3 times a day (sometimes 4 to accomodate for night shift workers) and it's free. Some days it lacks some of what one would need, like brown sugar for oatmeal (which lead to my discovery of "cookie oatmeal"). Most days its menu is fairly consistent, and some DFACs even have an outdoor barbecue area where there's a specific meal each day (big burger sundays, Mongolian barbecue saturdays, etc).
Needless to say, a free place to get okay-tasting food is actually a pretty good idea. Some days I measure time by how long it is until it opens
Update: For those interested, we pronounce it as if it were spelled "defac". It stands for dining facility (the military has an odd way of varying the amount of letters from a word to be used in acronyms, in fact, it has a pretty funny way with acronyms altogether).