Yummy means tasting very good. It is the describing word built from the noise yum, and it works in the normal adjective places: yummy cake, this is yummy, something yummy for dinner. You can also compare with it — yummier, the yummiest — though those forms feel even more playful.

The important thing is register. Yummy is warm, informal, and a bit babyish. It belongs to parents talking to children, friends messaging about lunch, food photos online, and menus that want to sound friendly. A restaurant review in a newspaper would say delicious, excellent, beautifully cooked — not yummy. So if a student writes "The chef prepared a yummy dish" in a formal piece, the grammar is fine but the voice is wrong, like wearing pyjamas to an interview.

It has stretched beyond food, the way tasty and delicious have. People call a soft blanket, a nice smell, or a piece of gossip yummy, meaning simply "lovely, I want it." Applied to a person it means physically attractive, and again that can be playful between friends or uncomfortable from a stranger. There is also the set phrase yummy mummy, mainly British, for a young mother who is fashionable and good-looking — a phrase some people enjoy and others find rude.

A gentler, very common near-twin: tasty. Same friendliness, sounds a year or two older.