A locksmith is someone that makes and repairs locks .A combination lock is a type of lock that uses a dial that spins in order to open it. The locksmith should have a "good combination of skills" meaning, they should have a varied skill set. The joke is clever in using the word "combination" as it can relate to a lock type. The joke is centered on the word "combination" and its different uses.
A locksmith has to have a good Combination of skills .