Calentura literally translates to "fever," like the high temperature you get when you're sick. However, in this song and many colloquial contexts, it's used to describe the "heat" of passion or intense desire.
Nicky Jam sings, "Una calentura sube la temperatura" (A heat raises the temperature), perfectly capturing that overwhelming feeling of attraction. It's a fantastic, expressive word that vividly describes being swept up in a moment of intense connection.
This song is pure summer heat! Nicky Jam and Beele are on the beach, and it's not just the sun that's making things hot. The whole track is about an intense, immediate attraction. Nicky Jam tells a woman, "Tú me calienta' más fuerte que el sol" (You heat me up more than the sun), which is a pretty bold way to start things off. It's a song about that feeling when you see someone and the chemistry is so strong it feels like a "problema" (problem), but the kind of problem you definitely want to have.
He's completely focused on her, complimenting everything from her dancing to her natural beauty, saying she's hot "sin tener que ir al gimnasio" (without having to go to the gym). There's also some classic reggaeton swagger here, especially when he compares himself to her current boyfriend with the line, "Yo soy un Richard Mille, el novio tuyo es un Casio" (I'm a Richard Mille, your boyfriend is a Casio). He's basically saying he's a luxury upgrade. It's a fun, flirty song about wanting to let the temperature rise and see where the day, and night, goes.