If you were accused of wrongdoing, you would gather compurgators to appear for you in court. Ideally, you would find 12 of the most respected members of the community who would be willing to stand there and say that they believe you when you say you are innocent. Mind you, if you were found standing over a dead body with a bloody knife in your hand, compurgators were not likely to save you. This worked well when you were accused of cheating on a debt or stealing a spoon and hard evidence did not exist against you...unless you had friends who were determined to protect you.
The opportunities for abuse of such a system were rampant.
Henry II, or instance, in 1164 made sure that compurgation would not be allowed in felonies; he did not like the fact that a cleric (priest) might literally get away with murder in an ecclesiastical court by merely being defrocked, while the royal courts would use capital punishment for capital crimes. The use of compurgation in any way as a defense in England was eliminated from the court system in 1833.