Old breaker needs replacing. Is this a 2 pole 40 amp breaker (40 each pole) or a 2 pole 20 amp breaker? (20 + 20 = 40).
It's a 40 ampere double pole breaker.
It can supply two 40 ampere 120 volt circuits, one 40 ampere 240 volt circuit, or any combination of the two.
It may seem complicated, but it's really quite simple. Go to the hardware or home improvement store, and ask for a double pole 40 ampere breaker.