Thrust Reversers. Thrust reversers redirect exhaust air from the engine in a forward direction, thus helping slow down the airline. Most modern airliners do not support thrust reverser use in-flight.
Thrust reversers generally provide no more than 20% of the braking action on landing. They are a valuable safety assist and help reduce wear and tear on the brakes.
Turboprop airplanes do not have thrust reversers, but the propeller blades can be rotated in a manner (called beta control) that approximates the same effect.