What is the meaning of "Defying Gravity" on Glee tv series?

It's a song from a musical called "Wicked." Look it up, it's pretty interesting. In the musical, it's sung by a girl who wants to defy what the institution (her school, the society she was born into) dictates. The same meaning applies in Glee, but with some added dimension. Kurt wants to sing "Defying Gravity" for two reasons: 1) a girl song performed by a male would give it an added edge and 2) Rachel Berry sings all the good songs so why not give him a chance to audition for it? Kurt doesn't end up with the song because he knows that his recent coming out would harm his father, which puts the whole "defying gravity" concept on hold for Kurt's purposes. He values his father more than he values pursuing his passion being Glee and fashion and all things bright and pretty. I like how Glee used this song in the show. It's an anthem for people who want to pursue their dreams, something that both Rachel and Kurt want. In this episode though, sometimes, we have put ourselves on hold for the sake of others.

Agreeing with the others, it meant defying the boundaries of society. His differences will not bring him down The sad reality subtext is that Chris Colfer wanted to sing this song at his school but was barred from doing so because of his gender. Just watch Lea and Chris perform. Lea sings if beautifully and effortfully, but there is no denying her competitive manner on her face and effort. But you will notice that Chris sings it so naturally. The show makes it painfully clear that Kurt was the ideal choice for the song. However, his identity also laid in his love for his father. We know what happens next. Although it did send a mixed message that he had to sacrifice it for his father, it really touches us with the extent of his devotion.

i think it means that the person singing it wants to fly .

Well originally it's from the musical Wicked and not from Glee directly. In Glee I really think it has more to do with Kirk then it does "I'm the focus of every frigging episode" Rachel. Kirk is trying to toss off the expectations of him as a boy becoming a man in high school. He's expected to act manly, not be comfortable with himself, and most importantly not be a homosexual. In this specific episode he is even denied the chance to try out for the lead on this song based solely on gender and not talent. So really defying gravity is just about defying the laws set around him and being himself. Part of me wishes he hadn't purposefully blown the last note, but I completely understand why he did and it lead to a fantastic character moment for the show.