Hunting Pressure is established whenever you fire and land a shot on an animal that is eventually downed. Other animals will start to avoid the area with hunting pressure. Hunting pressure can build in a single area, causing even more animals to avoid it.
Hunting pressure will disappear as time passes or as it appears elsewhere. It is illustrated by a colored (light violet to glaring pink) field on the map. Different intensities of pressure can be distinguished based on the shade of purple of a particular area.
Despite its ingame description, hunting pressure only appears if the shot animal dies, so it can be used to estimate if an animal is dead.