Plain English Meaning
The airplane is going down so fast, and is so low, that there is no longer enough height or power left to stop the descent before hitting the ground.
Full Definition
A rate of descent that has become so high, so close to the ground, that the airplane cannot recover before impact regardless of pilot input or available engine power. Once entered, the combination of altitude lost, energy state, and aircraft performance limits makes arresting the descent impossible in the time and height remaining.
Context Anchor
Seen in low-altitude maneuvering, approach, flare, and go-around discussions, especially when the airplane is slow or close to the ground.