Plain English Meaning
It is how far the airplane is from a chosen spot on the ground, measured along the ground itself. To keep that distance the same all the way around a turn, the pilot has to adjust the bank angle to account for the wind pushing the airplane.
Full Definition
The distance from a fixed point on the ground to the airplane's path over the ground, used as the reference for flying ground reference maneuvers such as turns around a point or a rectangular course. It is measured along the surface, not through the air, and stays constant only if the pilot actively corrects for wind.
Context Anchor
Used in ground reference maneuvers such as a rectangular course, where the pilot tries to keep the airplane the same distance from while turning around corners.