A meat anchor is a person used as an anchor. We use meat anchors all the time to keep our speed up or to help less competent downclimbers. We have two basic meat anchor setups that we use most frequently.
Setup #1: The competent downclimber acts as a meat anchor for less competent downclimbers. He usually sits down behind a pothole, rock, tree, or other stable feature, so he isn't pulled over the edge. We tie a figure 8 on a bight and clip it to the meat anchor's harness. Then the less competent downclimbers rappel down the rope (or downclimb with no hands on the rope while a belayer provides a fireman belay). Once everyone else is down, the meat anchor throws down the rope and downclimbs the obstacle (with a spotter if necessary).
Setup #2: For a low-angle slide or similar situation, we use a 30' piece of webbing with overhand on a bight knots tied every 12"-18" (for handholds). We have a large loop tied on the end. The meat anchor at the top puts one of his legs through the large loop and slides it up to his knee, then he kneels on the ground. This keeps the pulling force down low, so it doesn't tip the meat anchor over. Once everyone slides down with the handline, the meat anchor throws down the webbing and downclimbs or slides down (with a spotter if necessary).
Meat Anchors are great for helping less experienced or less athletic people down obstacles that can be downclimbed by more competent members of the group. This reduces the construction of unnecessary anchors, rope grooves, and other eyesores. Meat Anchors are very fast to set up compared to building or installing permanent anchors.
Meat anchors can be quite uncomfortable for the person acting as meat anchor, especially if the person rappelling is large or bounces on rappel.