From PackRat Recipe Wiki

A lock is a special card used to keep other items safe. If you have a lock in your pack the "Lock stuff" will be lit. Click on it and click the the item you want locked. Click the "Lock" button on the side and your card will be safe for as long as you have it locked.

To unlock a card select "Lock stuff" and click an already locked card. The button on the side will now read "Unlock".

If you lock an item up nobody can steal it. Locks are now unbreakable.