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 three basic meat anchor setups that we use most frequently.
Setup #1: The competent downclimber acts as a meat anchor for less competent downclimbers. They usually sit down behind a pothole, rock, tree, or other stable feature, so they aren't isn't pulled over the edge. We tie a figure 8 on a bight and clip it to the meat anchor's harness, ir we run the rope through their rappel device. 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 or partner capture 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 their legs through the large loop and slides it up to their knee, then they kneel 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 or partner capture if necessary).
Setup #3: We tie a rope or piece of webbing onto a pack. The meat anchor sits on the pack while the others rappel or handline down. The last person then downclimbs or slides down (with a partner capture or pack drag if necessary).
A meat anchor can also be used to send someone down a drop to search for materials to build an anchor when none exist at the top of a drop. Things like logs can be hauled up from the bottom of a drop to construct an anchor for the last person.
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.