We're down to earth
“Down to Earth” is an old idiom that means “to be realistic”.
What Katy is trying to say is that she and this boy know their limits and are realistic. They are certainly not caught up in an imaginary relationship.
Aforementioned, her love will make you fly, but they will come back down to earth if you choose not to love her.
To help improve the meaning of these lyrics, visit "Dark Horse" by Katy Perry (Ft. Juicy J) Lyrics and leave a comment on the lyrics box