Bandido literally translates to "bandit" or "outlaw". It's a word with a rebellious and adventurous feel.
In the song, KAROL G uses this word poetically to call out her ex, singing, "Bandido de los sueños que aún no hemos cumplido" (Bandit of the dreams we have not yet fulfilled). She's accusing him of stealing the future they were supposed to have together, making it a powerful and memorable way to describe a heartbreaker.
This song is all about that messy, post-breakup feeling where you're trying your best to forget someone, but it's just not working. KAROL G starts by saying she's had way too much to drink but still can't get her ex out of her head: "Si van como quince y no puedo olvidarte" (I've had like fifteen [drinks] and I can't forget you). The title itself is the most dramatic line of all: "Que cuando me muera te olvido" (I'll forget you when I die). She's basically saying she will never forget him.
But she's not just sitting around feeling sad. She's also remembering how good they were together and feels sure he misses her too. She reminds him, "Decías que yo era tu reina, que era tu bebé" (You used to say I was your queen, that I was your baby). Then she confidently tells him, "Tú matas por mí, baby, no te hagas" (You'd kill for me, baby, don't pretend). It's a powerful mix of heartbreak and knowing your own worth.