A goody is a special treat, especially one you can eat. Sometimes kids judge the quality of a birthday party by the goody bag they leave with.
When you're a kid, a highlight of other people's birthday parties is the goody bag, a gift of small toys and edible treats. If you celebrate Christmas, you probably get goodies in your stocking, and for Easter some lucky children receive baskets full of goodies. Goody, which since the 18th century has meant "something tasty," can also be used as an exclamation meaning "hooray:" "Oh goody, it's the ice cream man!"