Food is the resource used to feed monsters to increase their level, so they can become stronger and generate gold faster. Food is also used instead of gold to upgrade some buildings, like upgrading habitats or for upgrading relics.
Food is mainly grown on farms, but can also be earned by other means, like completing a level on the Adventure Map, or as a reward.
Like gold, players can purchase food using gems, which is purchased using real life currency.
Monster Level Up Costs
The amount of food needed to level up a monster depends on the level it is being leveled to, regardless of the rarity of the monster.