The simple answer: kefir is a semi-liquid cultured milk product that tastes really good with sugar. You can drink it from a glass -- flavored kefir is already very sweet. It is in some sense the Russian version of yogurt, with slightly different cultures. The Lifeway company, maker of the kefir I usually buy, has a really interesting history.
If you read the recipe in my earlier post today (about chocolate cake), you know that I've also identified kefir as a convenient substitute for buttermilk in baking. It's convenient because I'm much more likely to have it on hand than buttermilk, which I would buy only for baking. Also, I think it gives a better result than what I would normally use, which is ordinary milk with a few drops of vinegar or lemon juice.