Plain English Meaning
The drag the ground puts on the wind. Trees, buildings, hills, and even the surface of the ocean rub against the moving air, slowing it down and bending its direction near the surface.
Full Definition
The resistive force created where moving air contacts the Earth's surface, slowing the wind near the ground and altering its direction relative to the wind aloft. Friction is significant in roughly the lowest 2,000 feet of the atmosphere, known as the friction layer or boundary layer.
Context Anchor
Seen in aviation weather discussions about surface wind, wind aloft, and why wind near the ground may not match the wind above the surface.
Derivation
From Latin 'frictio,' meaning rubbing. The word still carries that everyday sense here: air rubbing against the surface it flows over.