Cooking is a Skill which you can buy from the Bookstore. You need to use it on one of Your Crew to learn the ability. Once you have learned the skill you'll need to place some Meat, Fish or Stew on your boat somewhere. The Pirate with the ability learned will automatically begin cooking it into something edible for the rest of the crew so long as they are in the Ship Crew group.
It is great in combination with fishing as this can provide a cheaper and easier food supply for the crew