Because zero gravity is fly but that's just basic physics
Gravity is a force that causes things to drop down to the ground. Without any gravity aka “zero” gravity" things would just float/fly around. It’s basics physics because it is one of the beginning fundamental things you learn about in Physics class.
Also, this is a reference to the mixtape this song is on: “Zero Gravity”. He’s saying this mixtape is fly aka extremely good, but that just goes without saying because it’s him spitting.
To help improve the meaning of these lyrics, visit “Turnt Up” by Los Lyrics and leave a comment on the lyrics box