The main use was answered. You may also see it used for inventory for items that "stack" together. For example, each Gysahl Green doesn't take up a separate inventory slot. Any Gysahl Greens you have in inventory will try to condense into a single inventory slot, which is then referred to as a "stack of Gysahl Greens". Inventory slots can only hold up to 99 copies of a stackable item so you may sometimes see a "full stack" used to refer to 99 copies of that item.