Its a measure of efficiency to allow people to compare the energy consumption, they're rated in MPGe (Miles Per Gallon equivalent)

Why list it as Miles per Gallon instead of kWh/mile or miles/kWh? Because they did some surveying and determined people don't know what a kWh means in terms of their car and found MPG less confusing and consistent with the existing markings

121/91 MPGe city means the Leaf used 0.278 kWh/mile on the city driving test and 0.370 kWh/mile on the highway test

It doesn't directly compare to the costs/CO2 emissions of a gasoline powered vehicle because it doesn't deal with the upstream power generator efficiency but its a start and allows for comparisons of efficiency between electric cars beyond just the stated range which is heavily influenced by overall battery capacity