It’s MPGe (miles per gallon equivalent), and it was invented so that consumers could (sort of) compare the efficiency of electric cars and non-electric cars. Unfortunately, it’s not intuitive in any way.

The US Environmental Protection Agency determined that 33.7 kWh of electricity was equivalent to 1 gallon of gas. So, if a car goes 4 miles on 1 kWh of electricity, that’s equivalent to 33.7 x 4 = 134.8 MPGe

The 33.7 kWh per gallon assumes 100% efficiency in conversion of the chemical energy of combustion in a gallon of gas into mechanical energy. However, a typical internal combustion engine is typically about 25% efficient (and electric motors close to 98%). As a result, it’s a pretty dubious way to compare EVs and non-EVs. It’s an OK way to compare EV to EV, though miles per kWh is more meaningful because it can be easily used to estimate the cost of operating the car.