Cooking is the 7th and last skill to be unlocked. It is unlocked by crafting an oven, which allows you to cook fish. Fish is passively obtained by the Fishing Rod, using up Fishing Bait for each fish caught. Fishing Bait can be obtained through Farming after unlocking the second Research Perk. Eating cooked fish and other food-items gives you Energy, which is mandatory to use the Exploring Skill.
To cook foods, you need heat. The amount of heat you need to cook a food is next to the items in the tables above. To add heat, click on your oven and tap on the logs you want to add to your oven. Then to cook the food, just tap on the food you want to cook, enter how many of that food you want to cook, and tap cook. Congrats, now you are cooking some food!