Plain English Meaning
It is the last full check the pilot makes before lining up for takeoff, confirming the engine and the airplane are ready to fly safely.
Full Definition
A systematic procedure performed after taxiing to the runup area and before entering the runway, in which the pilot verifies that the engine, flight controls, instruments, and aircraft systems are functioning properly and that the airplane is configured correctly for departure. It typically includes an engine runup, magneto check, carburetor heat or alternate air check (if applicable), control surface free-and-correct check, instrument settings, fuel selector , , , and a of items.