Much like Silver, Food is one of the most important resources in the game for strengthening your defenses. Food is used to train certain troops and to maintain your army. It can also be converted into Religious Tractates in Clan Temples. Finally, Food can be used to charge a Clan Capital's Shield of Peace.
Build and upgrade Farmers' Houses in order to increase production of this resource. You can also gather resources on the Kingdom Map or obtain them from Clan gift chests or chests that can be purchased in the game's shop. Finally, players can exchange resources with one another if they have built the Caravan. You can read more about this and other buildings here.
All troops in your City (with the exception of allied reinforcements) consume food. Please note that if the amount of Food being produced is lower than your army upkeep cost, then your Food counter will turn red, and your City will receive the Food Deficit debuff, which increases the cost of training troops. The greater the deficit, the greater the cost increase. If the amount of Food in your City reaches zero, your City will receive the Famine debuff, which decreases your army's strength and health by 25%.