A Hold can be called by anyone (including spectators) as a safety measure during combat. If anything unsafe is seen, such as part of armour coming loose, a sword breaking, or a dog/small child running onto the field, someone drawing a weapon without calling "clear" the word HOLD! must be called.
Upon being called, all SCA combatants must drop to one knee and repeat the call (so those who may not have heard it hear it). If you are an archer, you must un-nock any arrows, and hold your bow above your head. If you are a heavy, your weapon must be held high above your head.
These actions apply to a 'Hold' call during war and tournament fighting, and target archery/combat archery. If you hear a 'Hold' at any other time, freeze and work out who made the call and what the hazard is before taking any further action.
Please note: If you see anything that you think might potentially be a dangerous situation, do not hesitate to call a Hold yourself. It is much better to be wrong, and a bit of time wasted, than to be too embarrassed and let somebody get hurt.