Food is a static resource in the Apocalyptic Dawn techtree. Most units require it in at least a small amount. Generally, infantry units require 1 unit of food (though some larger and stronger ones require 2 units) whereas vehicles manned by multiple people tend to require more, up to 3 units each.
Food is a static resource, so cannot be harvest from any part of the environment. Instead, it is produced from the Rations Center in UNATF in quantities of 15.