A food ration is a generic term for an amount of food assigned to you when supplies or space are limited. I imagine there are two types of "food rations":

1. Where all the food in an area is being rationed due to some interruption in supply, like an earthquake, a plague, or a war. In this case, your "food ration" would probably consist of various types of food, like milk, bread, sugar, beans, fruit, etc, but in limited quantities. So it would still taste ok, but you'd have to be careful about not wasting it.
2. Alternatively, a package of not-very-tasty-but-hopefully-nutritious "food" you are given for travel or emergencies. Hard-tack is one example -- it's a brick of a baked water/flour mixture (with possibly some other stuff thrown in) that is compact and won't go bad. Not a culinary delight, but it'll keep you going. I saw some hard-tack for sale at a souvenir shop once. After squeezing it, I decided to spare myself a hefty dental bill and not buy it.

In NetHack, I suppose a food ration is more like the second case above. It's worth 800 nutrition points, like lembas wafers, and will cost you around 60 zorkmids in a shop (unless you actually need one, then it will cost much, much more). And no, you can't sacrifice a food ration on the altar anymore to instantly max out your alignment; they fixed that bug years ago.