It's used to separate the fat from the juices after the meat is cooked. Usually so when you make a gravy or au jus, it isn't overladen with fat.
You can find them at any store that carries housewares. Probably Wal-Mart or Target. Bed Bath and Beyond should carry them as well, or any store like that.
I've attached a picture so you know what to look for.
The goal is to skim most of the fat off of the pan juices. One device that does this looks like a measuring cup with a spout connected near the bottom. Fat floats on top, so the liquid poured out via the spout is mostly juices, leaving the fat behind.
You can get the same effect by pouring the juices in a cup, waiting a few minutes for the fat to rise to the top, and skimming it off with a spoon. I like to use Chinese soup spoons for this.
If you are making the dish a head of time (say a stew) the best way of removing the fat is letting it solidify in the fridge. A good general cookbook should have various suggestions on how to skim fat off. A special device may make it easier, but you don't have to buy one.
Had one of those measuring cups with the low spout but it broke when I put the hot pan juices in it, duh, I agree with Paulj, there are ways around it, chill the stock if you have time, a bulb baster works if it is still hot, just suck the stock off the bottom of your 2 cup Pyrex you have dumped the juices into, I always have trouble skimming with a spoon the fat off hot juices.