Calentito is the diminutive form of caliente (hot/warm), meaning "warm and cozy" or "nice and warm." It adds a sense of comfort and endearment.
In the song, the speaker defiantly declares, "Porque este invierno yo mi amor la paso calentito" (Because this winter, my love, I'll spend it warm and cozy). It's a playful yet firm rejection, implying they've moved on to a more comfortable and happier situation without their ex.
Alejandra Guzmán opens this duet with Farruko already past the point of pretending: "Prefiero no disimular, decírtelo al oído" (I prefer not to hide it, to tell you in your ear). She's decided the relationship is over, and every repeat of "Adiós, (ay no me hagas esto amor)" (Goodbye, (oh don't do this to me, babe)) is her closing the door a little harder each time he pushes back.
Farruko's verse is the other half of the same fight, and he takes the pleading role: "Si quieres me pongo de rodias" (If you want I'll get on my knees), "Te compro lo que quiere, lo que tú me pidas" (I'll buy you what you want, whatever you ask me for). But he also admits what got them here — "Me comporto como un perro... pero así me conociste a mí" (I behave like a dog... but that's how you met me) — which is exactly the excuse she's done accepting. By the closing "Te estas tardado mi amor" (You're taking too long, babe), the goodbye has already happened; he's just the last one to notice.