It's a playful and slightly flirty expression, used to compliment someone on their attractiveness. It can be used both romantically and platonically, depending on the context and the relationship between the speakers.
The phrase is often used with a playful tone and a smile, emphasizing the "qué" (what) for added effect.
Here are some examples of how it could be used:
* "Y si qué guapo eres, Juan!" (And what a handsome you are, Juan!)
* "Mira, ya te dije, y si qué guapo eres." (Look, I already told you, and how handsome you are.)
* "Me encanta tu camisa, y si qué guapo eres con ella." (I love your shirt, and you look so handsome in it.)