Chest locks are used to keep client players in multiplayer from opening your chests. Chest locks will also keep cliant players from destroying your chests and collecting the items inside.
You (the world owner/host) can still access your chests normally.
Then on a chest a lock and iron band are visible on the chest (like the chest on the right, seen above)
To put a lock on a chest, just hold the lock in your hand and tap on a placed, storage chest.
Keep in mind, once you put a lock on a chest, the only way to get it off is to break the whole chest, but that's not much of a problem.
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