While an HVAC contractor may know that a four-ton air conditioner does not in fact weigh four tons, but rather represents the capacity of the unit, there is a sense of consumer confusion on the subject. Put simply, the measurement represents the amount of heat an air conditioner can remove from a space in one hour. This grows more complex when AHRI derating and nominal vs. actual capacity of a unit come into play, but essentially this is a serviceable definition. A four-ton unit removes 48,000 BTUs (British Thermal Units) of heat per hour, with each BTU being equivalent to the amount of heat released by a match burned all the way down.
Read the full article here: Why Do We Measure Air Conditioner Capacity in Tons?