Boundary Layer Ingestion (BLI) is a promising idea NASA researchers are studying to reduce fuel burn in jet engines, thus reducing emissions and the cost of operating the aircraft. Thrust makes an airplane go forward, while drag tries to slow it down. Lift offsets the weight to keep an airplane in the sky. BLI deals specifically with the drag part of the equation by trying to reduce the total drag an airplane experiences in the sky.
When the airplane’s engines are put at the extreme rear of the airplane, the slower, boundary layer air is ingested into the engine and is then accelerated with the rest of the air passing through the engine and exhausted out the back. With less total drag, the engines need less thrust to push the airplane forward, which means they don’t have to burn as much fuel.