it's a comical way to say "keep going" or "push through" or whatever. its supposed to be an encouraging quote and usually serves its purpose, also making that person smile
"dude i've been so sad. i can't believe he left me for that other girl"
"hey, keep your head up princess, your tiara is falling"
A sly way of calling a girl as perfect and graceful as a princess when cheering her up. An encouragement to just keep standing proud.
Girl: Ugh, literally nothing is going right today. I fell flat on my face in the audition, and my best friend is not talking to me anymore. I just feel so awful.
You: Oh that's awful! You deserve way better than that. Just Keep Your Head Up Princess, Your Tiara Is Falling
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.