Suppose you want to send in the mail a valuable object to a friend. You have a box that is large enough to contain the object and the box can be fitted with multiple locks.
Both of you also have several locks and their corresponding keys. However, your friend does not have the keys to any of your locks. If you send a key in an unlocked box, there is a danger of theft.
How can you send the object securely to your friend?
Put the valuable object into the box, secure it with one of your locks, and send the box to your friend.
Your friend should then attach one of his own locks and return it back to you.
When you receive it again, remove your lock and send it back. Now your friend can unlock the box with his own key and retrieve the object.