Something that's baked is cooked in a hot oven. Baked potatoes are especially delicious when they're covered in melted butter.
If you agree to supply baked goods for a school bake sale, it means you've pledged to spend some time mixing batter and dough and then baking the results in an oven. You could eat baked fish for dinner and follow it up with a freshly baked apple pie for dessert. The adjective baked comes from the verb bake, from the Old English root word bacan, "to bake."