Frontear is a classic Spanglish verb popular in reggaeton, adapted from the English phrase "to front". It means to show off, act tough, or boast about your status and lifestyle, often in a defiant way.
In the song, Bad Bunny feels responsible for his ex's new wild attitude. He sings, "Yo fui el que te enseñé... a frontear como es" (I was the one who taught you... how to front properly), revealing that he taught her the very behavior he now dislikes seeing in her.
Bad Bunny sounds completely confused and a little hurt in this one. He starts by asking his ex, "¿Cómo fue que terminamos siendo enemigos?" (How did we end up as enemies?). He feels like he didn't do anything wrong and that just a month ago, things between them were great. He's looking at her new life and doesn't even recognize her. He claims she wasn't like this when they first met and even takes credit for her transformation, saying, "Siento que yo te cambié / Después que te lo metí" (I feel like I changed you / After I f*cked you). It's a classic case of a guy who liked a 'good girl' but now can't handle the wild side he thinks he unlocked.
He paints a super specific picture of her new party-girl lifestyle. She's out at the club to "perrear" (dance reggaeton), wearing a "trajecito sin panty" (little dress with no panties), and posting stories of herself "twerkeando como Cardi" (twerking like Cardi B). He notices she's looking for a rich guy to buy her a Rolex or a Cartier watch. He seems to be watching her every move, from her friends to her drinks ("Por la noche bebiendo tequila, por el día matcha" [Tequila by night, matcha by day]). By the end, he's just annoyed, telling her "ya tú no me caes bien" (I don't like you anymore) and admitting he's jealous and hurt to show his feelings to someone who "no lo merece" (doesn't deserve it).