Vago is a fascinating word with a double meaning. While it can literally translate to 'lazy' or 'bum', in this song it's used as slang for a 'wanderer' or a 'player'—someone who's always out on the streets and not tied down.
He sings, "Tú me conociste bien vago" (You knew me as a real wanderer), to contrast his wild past with how completely infatuated he is now. This word is key to understanding the song's popular 'reformed bad boy' story.
This song is a super direct and possessive love song. The singer tells his girlfriend, "Mija, arréglate, ponte bonita" ("Honey, get ready, make yourself pretty") because he's coming to see her. He's not shy about it either, telling her to warn off any other guys interested in her: "avísale a tus pretendientes" ("let your suitors know"). The chorus is just him repeating over and over, "Y solo tú, tú, tú, tú / Me perteneces" ("And only you, you, you, you / You belong to me"). It's intense!
He also mixes romantic details, like giving her flowers, with a much tougher side. He mentions her innocent social media image, "Toda una santita en las redes" ("A total little saint on social media"), right before a line about not being scared if he pulls out a rifle. He even tells her, "No te asustes, mija / Ya sabes que tú eres mía" ("Don't be scared, honey / You already know that you're mine"), which is a pretty bold thing to say. It's a song about total devotion, but with a definite edge.