A quote meaning to "Be proud to be a girl". Also saying that "Boys aren't better then girls". It is a sign of showing how girls should be treated equally.
Girl: I'm a GIRL and I'm PROUD! Boy: No, girls are nothing. You're all lower class! Girl: Actually, studies have shown that girls are smarter than boys. Boy: *Is mind blown*
It's the expression that you use to secretly name your husband/boyfriend when you both are forcefully closeted and you have to pretend to have a female girlfriend.
On the Miss You promo, Louis discreetly explained that the song was about the feeling of missing his husband Harry:
"...it was always in the back of my mind that what I really missed was the girl that I loved."