"Guapa" is a Spanish word for "pretty," but the "-ita" makes it a term of endearment. So, you'd only say it to/about a pretty girl you really care about.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.