means A lightweight jacket designed to resist wind and light rain, worn for outdoor activity rather than serious cold or storms.

from A transparent English compound of "wind" plus "breaker" — literally a garment that breaks the force of the wind. "Windbreaker" became a genericized term in American English (where it overlaps with the British "anorak" and the trademark "Windcheater"), built on the much older sense of "break" meaning to lessen or blunt a force, the same logic behind a "breakwater" stopping the sea or a "windbreak" of trees sheltering a field.