Nalgotas is a colloquial and somewhat exaggerated term for "big buttocks" or "a large behind." It's derived from nalgas (buttocks) with the augmentative suffix -otas.
In the context of this reggaeton song, it's used to describe the prominent physical attribute of the women dancing, emphasizing their curves and the way they move. It's a vivid and common slang term in urban music, making it memorable and relevant to the song's theme.
Wisin announces the record like a fight card: "Este es un corte explosivo, Jowell, Doble con La Nota" (This is an explosive cut, Jowell, Doble with La Nota). What follows is perreo built around one instruction, repeated until it stops being a word and turns into a drum: "Mueve, mueve, retumba cuando se mueve" (Move, move, it rumbles when it moves). The verses are boasts about a woman who owns the dance floor, and the line that makes her a person rather than a body is about her shoes: "Sencillita, a la discos llega en Crocs" (Low-key, she shows up at the club in Crocs). She has nothing to prove to anyone in the room.
The better lines are the ones that drop the volume. Wisin credits his producer and then slows the whole thing to a simmer: "Jumbo pone la base y yo lentamente la cocino" (Jumbo lays the beat and I cook it slowly), "A fuego lento con una copa de vino" (On a low flame, with a glass of wine). That is a strange image to find in a song this loud, and it works. Jowell y Randy take the back half and shove it into noise again, and the track signs off with a studio roll call, "Directamente desde La Base" (Straight from La Base). If you are learning Spanish, this one is thick with commands and with Puerto Rican street pronunciation: mueve, dale, muevete, plus clipped endings like pa' for para and to' for todo that reggaeton uses constantly and textbooks never teach.