here's the answer:

1. This song is about striving for the dream no matter what other people say.

2. The purpose of this song is to motivate one self to try to make one's dream come true against all odds.

3. The message is not to give up the dream or get demotivated by other people's opinion.

4. The word ' you' here refers to other people who look down on us.

5. The meaning of a phrase 'defying gravity' is the attempt to challenge all the negativity around you which could demotivate you to make your dreams come true.