The song is called "I Got Five On It."

Which is five words long and also includes the number 5.

Five squared is 25.

Y is the 25th letter of the alphabet.

By including it in the trailer (and not just including it, but foregrounding it,) he's really using it to ask "why," as in "why do the absolute dumbest people on the internet insist on projecting their own weird bullshit on to a trailer for a movie they'll never see?"