Plain English Meaning
Speedbrakes are flat panels the pilot can extend into the airflow to make the airplane slow down or come down faster, without having to pull the throttle back.
Full Definition
Speedbrakes are aerodynamic devices, usually deployable panels on the wings or fuselage, designed to increase drag without significantly changing the lift the airplane is producing. They allow the pilot to slow the airplane or increase the rate of descent without reducing engine power.
Context Anchor
You may see this term in stall recovery guidance, descent planning, and aircraft operating procedures for airplanes equipped with these panels.