Trato translates to "deal" or "agreement". While it's a common word, Maluma uses it here to describe a very unconventional romantic arrangement.
He sings, "Yo te acepto el trato" (I accept the deal), laying out the terms of an open relationship where both partners are free to be with other people. This word is the key to understanding the song's provocative theme of being "Felices los 4" (Happy the 4 of us).
Colombian superstar Maluma teamed up with salsa legend Marc Anthony for this version of his hit song, and it's all about a very modern, very open relationship. The singer tells his lover that he knows she leaves him as soon as the sun comes up, but he isn't worried. He's so confident she'll come back to him that he says, "Si conmigo te quedas / O con otro tú te vas / No me importa un carajo porque sé que volverás" (If you stay with me / Or you go with another guy / I don't give a damn because I know you'll come back).
He doesn't just tolerate the idea of her being with someone else; he actively encourages it with a shocking and funny proposal. He suggests that if she's with another guy, they should all just be happy together. The chorus hits with the famous line, "Vamos a ser feliz, vamos a ser feliz / Felices los 4" (Let's be happy, let's be happy / The four of us happy). He even offers to "Te agrandamos el cuarto" (make the room bigger for you), inviting her new partner to join them. It's a song that throws traditional ideas about love and jealousy out the window, focusing instead on a mutual agreement where nobody gets hurt and everyone has a good time.