An adjective. Often used to describe a woman. Someone with a pure, friendly, and calm face. Their facial features are usually delicate and soft. A person with a sweet face isn't pretty or cute... They are beautiful.
Dan: Did you see that waitress at that coffee shop?
Tom: No. Why?
Dan: She had such a sweet face. We 'ought to get coffee there one day.
Tom: Whatever, man.
Someone who says or does something that is so ridiculous that you literally have no way of expressing your feelings except shouting out a random phrase, thus "sweet face".
Guy 1: *says something really gay*
Guy 2: Ooo, yeaah, sweet face!
Guy 1: *Pops his collar*
Guy 2: You have a sweet face. Seriously.