What does it mean when you dream about locked boxes?

Containers such as boxes usually represent security, and in this case as they are locked there is an element of secrecy.

This may be interpreted as emotional baggage locked away. You may be carrying more difficulties than you like and have "locked away" some of them to make your situation easier to handle.

My question would be, "Do you have the keys?" If you do, then you have the control here and may start unlocking the boxes one at a time as you feel able to handle them. If you do not have the key, your subconscious may be hiding things from you that you don't believe you can handle just yet.