A car, truck, or other vehicle that is very "fuel-efficient" is able to drive for a long distance without using a lot of gasoline.

When people are talking about a vehicle's "fuel efficiency" in the U.S., they usually discuss how many "miles per gallon" it gets. That means how many miles the vehicle can drive using just one gallon of fuel. In other countries, distance is measured in kilometers and liquids are measured in liters. So you would talk about "kilometers per liter" instead.