It's a safety procedure: if equipment is faulty, you lock the controls with a special device to render it unusable until it is serviced, and a tag accompanies the lock to show when the service call was placed. If locking is impossible, just the tag will suffice.

LOTO or lock out, tag out is a safety practice of physical locking out any and all energy sources attached to a piece of equipment. Gravity, electrical, chemical, potential, pneumatic, hydraulic. You put a lock on it with a tag stating your name, date and typically a reason. You keep that key for that lock so no one else can energize.