It means that pool A has 60% of the capacity of pool B -- for example, if pool B has a capacity of 100 gallons, then pool A has 60% of this, or a 60 gallon capacity.
To solve this, we first need to remember that it's asking about the actual water in each rather than the capacity of each. We might have a situation where each is only partially full.
Statement 1 tells us how full each pool is, and we knew from the statement that B was bigger, but we still don't know actual gallon volume - we might have 6 and 10 gallon pools, or we might have 600 and 1000 gallon tanks. This is insufficient.
Statement 2 tells us the capacity of tank B. From this statement and the question above, we can determine the capacity of both tanks, but we don't know how full each tank is. Therefore, this one is also insufficient.
Together, however, we can determine the capacity of each tank (stmt 2) and the number of gallons of water in each (stmt 1) so together, we have sufficient information and our answer is C.