After the disappearance of the Harvest Goddess, the world is stuck without many crops; such as cabbage, tomatoes, and carrots. However, before the Harvest Goddess disappeared from this world, she imbued the Harvest Wisps with the knowledge of various seeds, thus ensuring the various fruits and vegetables of the world would not be lost forever. Using the power given to these Harvest Wisps, you'll need to unlock various seeds as you progress through the game! (harvestmoon.fandom.com)