Veneno literally translates to "poison". It's a powerful and dramatic word used to describe something that can harm or kill you.
In this song, Anitta uses veneno as a metaphor for her irresistible and dangerous allure. She's warning her lover that she's addictive and captivating, like a sweet poison they won't be able to get enough of.
Anitta is not playing around in "Veneno"! This song is a straight-up power move. She lays out the rules for her lover from the very beginning: "Si tú te portas bien, yo te lo voy a dar" [If you behave well, I'm going to give it to you]. She's in complete control of the relationship, and she knows it, even if he doesn't quite get it yet.
She compares her effect on him to poison and fire, which sounds intense, but it's all about her intoxicating power. She warns him directly, "Estás jugando con fuego / Cuidado, que te quemo" [You're playing with fire / Be careful, I'll burn you]. She's confident that he's more invested than she is, singing that he thinks of her more than she thinks of him. It's a bold declaration of female dominance and sexual confidence, where she is the one setting the terms and enjoying the game.