One of the core game loops in Sunflower Land requires you to feed your bumpkin so they can gain experience (XP). This is achieved by using crops that you have harvested to cook food at one of the followingBuildings. Better buildings require a higher level Bumpkin before you can use them.
Every farm starts with a Firepit so you can cook as soon as you have crops. Additional buildings can be found from the building menu by clicking on the hammer icon. Once you click on a building you can choose the recipe that you which to cook. It will then tell you what the food is, what ingredients it requires, how much XP you will gain and how long it will take to prepare. Once the food is ready you can click on the building to move it from the building into your inventory.
If you click back on the building whilst it's cooking you will see a timer so you know how long you need to wait.
Once the food is in your inventory you can click on your Bumpkin NPC on your farm and feed him.