Flavorless food tastes extremely bland. Sometimes when you're sick, all you want to eat are flavorless things like unsalted Saltine crackers and clear broth.
If you taste something and it has no flavor at all, you can describe it as flavorless. When food has spice, seasoning, sweetness, or any other distinct taste, that's flavor. Flavorless things are missing this essential aspect — think of a glass of skim milk, a bite of unbuttered white toast, or an unseasoned scrambled egg. You can also use this adjective in a figurative way, for anything lacking interest: "I stopped reading the book after two chapters of flavorless prose."