Plain English Meaning
The air an aircraft flies through is itself moving. That moving air carries the aircraft along with it, so the path the aircraft actually traces over the ground is different from the direction its nose is pointing, and the speed it makes over the ground is different from its speed through the air.
Full Definition
The influence of moving air on an aircraft's flight path, groundspeed, and heading. Because an aircraft flies through a moving body of air, any wind that is not directly aligned with the aircraft's nose will push it off its intended track over the ground, change how fast it covers ground, and require the pilot to compensate by adjusting heading or accepting a different groundspeed.
Context Anchor
Seen in navigation, flight planning, takeoffs and landings, and any time a is trying to on a chosen path over the ground.