A case to store your underwear in...

A case to store your underwear in is a briefcase.

This joke is a pun. One type of underwear is "briefs", and so a case to store briefs could potentially be called a briefcase. The problem with this is that there is already a object called a briefcase, and that object is used to store briefs (which are documents). Since the word is being redefined to a previously unknown (and confusing) definition, it is funny.