Plain English Meaning
It is the extra pulling or pushing power the engine and propeller have after overcoming the air resistance that holds the airplane back. That leftover power is what lets the airplane climb or speed up.
Full Definition
The amount of thrust available beyond what is needed to maintain level, unaccelerated flight at a given airspeed. This surplus is what makes climbing and accelerating possible — it is the thrust left over after drag has been balanced.
Context Anchor
Seen in climb performance discussions, especially when explaining why an airplane can climb better at some speeds and conditions than others.