얼굴 is face, and 피다 means 'to bloom.'
So if you translate it directly, it will be 'a face is blooming.'
Obviously, a face can't bloom.
Can it? Maybe.. lol
So if someone says, 너 얼굴이 폈다(Neo eolguri pieota). That means you look happier and brighter.
Example: 너 요즘 좋은일 있니? 얼굴이 폈다. (Do you have anything good happening? You look brighter.