Facing the direction from which the wind is blowing / eminating. Often used in relation to animal scenting and hunting. Hunters prefer not to be revealed by scent and therefore want to be downwind from a hunting source - the hunter can detect scent but cannot be detected if downwind.

Wild Turkeys Feeding in Heil Ranch Boulder

Hunting Wild Turkeys requires masking scent - being upwind would reveal scent to the animal.