Food is one of the many things Settlers need to survive in timber and stone. When your people's hunger bar gets too full, they will autonomously eat food from your storage, if any. When anybody's hunger bar fills up completely they will die of starvation.
A small percentage of your total food will be lost to rot over time. This means the higher the amount of food in storage the higher the rate of food you will lose to rot.
Rot makes large food stockpiles quite wasteful. A steady source of food is very desirable as it minimizes these losses.
Food can be gathered from the following sources:
- Fisherman gather food by fishing with a Fishing Rod.
- Farmers can make food by Farming. Farmers won't actually produce food until harvesting fully matured Corn, Carrots, Potatoes, Pumpkins or Turnips with a Hoe.
- Foragers gather wild berries and as well as hunt wild animals using a Knife or bow. Foragers can also gather wild Wheat but (again) is it not edible.
- Herders can gather food by slaughtering domesticated animals with a Knife.
- It's important to note that Wheat is not food and won't be eaten by hungry Settlers. In 1.71 there is no way to craft Wheat into food.
- Food is a fairly poor trade item as your Farmers could alternately be producing Wheat, Flax or Cotton, which doesn't rot and can be used to make more valuable goods.