As mentioned in the needs section the character needs to periodically eat to function normally. To do so press Space while holding a food item to eat that item and reduce your hunger.
Certain items that are not edible raw can be cooked into edible items. This includes corn, and all fish except minnows. These items are cooked by facing a stove while holding the item to be cooked and pressing E. The cooked form of the item will appear in your backpack, and can be eaten, stored, or sold. Cooked items generally sell for less than their raw forms since you don't run a restaurant.
Corn can be eaten once cooked.
Tomatoes can be eaten raw, and cannot be cooked.
Carrots can be eaten raw, and cannot be cooked.
Potatoes can be eaten once cooked.
Salmon can be eaten once cooked.
Bass can be eaten once cooked.
Tuna can be eaten once cooked.