Plain English Meaning
A fabric tube on a pole at the airport that shows pilots which way the wind is blowing and roughly how strong it is. The wind blows into the open end and pushes the sock out behind it.
Full Definition
A fabric cone or sleeve mounted on a freely rotating pole at an airport, used to indicate surface wind direction and provide a rough estimate of wind speed. The open end faces into the wind, so the sock points downwind. The angle the sock makes relative to its pole gives a visual estimate of wind strength: fully extended horizontally indicates a strong wind, while a limp or drooping sock indicates light or calm conditions.