Reaching Perfection in Stardew Valley means completing 100% of the game's content. If players are able to achieve this, the game will unlock several end-game bonuses. You shouldn't confuse "Perfection" for the Statue of Perfection that Grandpa may award you at the end of your second year. Instead, Perfection is part of the Stardew Valley 1.5 update that added Ginger Island and the Beach Farm. You can see how far along you are by checking the Perfection Tracker statue in Qi's Walnut Room on Ginger Island. You can only access this room after obtaining 100 Golden Walnuts.