What does the Korean grammar 아/어 버리다 mean?

Korean Grammar: 아/어 버리다 (Completed action (with feeling)) — Explained

Expresses that an action has been completed — often with a feeling of relief, regret or finality.

Tip: 잊어버리다 and 알아버리다 are common lexicalised forms — learn them as single words.

Korean	English
다 먹어 버렸어요.	I ate it all up (and now it's gone).
숙제를 해 버렸어요.	I got the homework done (finally).
잊어버렸어요.	I forgot (completely).