A zone is a grid on the Map. Every zone has a biome. Zones will give different items depending on the biome. If a zone is depleted, a red x appears on the zone in the map, it will not give any more items. A depleted zone will change to the next biome overnight. Frozen Soil(the lowest biome) will remain depleted forever. Zones that are the same will be located in around the same area of the map, so Forests is grouped together and Lakes are grouped together.
Zones can be instantly depleted for 5 Stamina. It (usually) costs less stamina to deplete a zone this way, but you only get half the items you would have gotten. This function is not present in tutorial games.
The zone color on the map.