A more thorough explanation:

A fire ordeal is a primitive form of trial in which an accused person is subjected to a dangerous or painful physical test, with the result being considered a divine revelation of the person's guilt or innocence. The accused person is usually subjected to holding a piece of hot metal or walking barefoot across a hot surface. The judgment of guilt or innocence depends on how quickly and cleanly the person's hands or feet heal. Typically, the person's hand is bandaged and, upon the bandage's removal three days later, is examined for festers (indicating guilt).

For example, in England, a piece of iron was put into a fire and then in the party's hand. The hand was bound and inspected a few days later. If the burn had festered, God was taken to have decided against the party.

The fire ordeal was commonly used in Europe until the 13th century, but only sporadically after 1215, when the Fourth Lateran Council forbade the clergy from participating in ordeals.