Nel is a very popular slang word in Mexico that means "no," but in a much more casual and informal way, like saying "nah" or "nope" in English.
In this song, which is even titled "NEL," the singer uses it repeatedly to brush someone off. He sings "Hoy pensé en salir a buscarte, pero nel" (Today I thought about going to look for you, but nah), perfectly capturing a cool, dismissive attitude and making it a key part of the song's catchy hook.
This song is all about trying to move on but failing spectacularly. The singer starts by telling himself he's over someone: "Hoy pensé en salir a buscarte, pero nel" (Today I thought about going to look for you, but nah). He's living the high life, moving from "Otra noche, otro hotel" (Another night, another hotel) and surrounded by women attracted to his expensive watch—"La enamoró mi Cartier" (My Cartier made her fall in love). He keeps saying "nel" to all these other beautiful women, trying to convince himself he doesn't need that one person.
But then the real story comes out. Despite all the partying, the money ("Pieles verdes, esta loquera me encanta" [Green skins (dollars), I love this craziness]), and the wild nights, he can't get her out of his head. He admits, "Baby, es que me perdí en tus ojos" (Baby, it's that I got lost in your eyes) and "Desde esa noche, mami, me tienes bien loco" (Since that night, baby, you have me really crazy). It's a classic push-and-pull between a rockstar lifestyle and being completely hung up on one person.