A pocket is an opening sewn into a variety of clothes (also backpacks and bags) for the storage of small items one does not wish to carry in the hands.
Although the derived verb, to pocket literally means "to put into one's pocket" its common usage refers to the act of stealing or keeping for oneself. To pocket something in this sense is to break one of the commandments.
The word pocket can also be used to describe distinct isolated areas within a whole. For example, air pockets in a pumice stone.