Lets simplify this down a bit. Lets just say that the pump only moved 1 gallon of water per second. There are 60 seconds in one minute, so it would be moving 60 gallons every minute. However, in this example the pump is moving 2 gallons of water per minute, so multiply the 60 by 2.
60 * 2 = 120
120 gallons of water are pumped every minute