Plain English Meaning
It's the pilot's careful look-over of the airplane before flying it — walking around the aircraft to check for anything wrong, missing, or unsafe.
Full Definition
A systematic visual inspection of an aircraft conducted by the pilot before flight to confirm the airplane is in a safe and airworthy condition for the intended operation. It typically includes a walk-around check of the airframe, control surfaces, landing gear, powerplant, fuel and oil quantities, and the surrounding ramp area, performed in accordance with the manufacturer's checklist.