Juras is the present tense of the verb jurar, which means "to swear" or "to make a solemn promise". It's a word packed with drama and high emotional stakes.
In the song, Luis Miguel's ex-lover returns and "juras que has cambiado" (swears that you have changed). This one word tells a whole story of betrayal and a desperate plea for a second chance, making it much more powerful than simply saying "you say you've changed".
This song is the ultimate breakup anthem for when your ex comes crawling back! Luis Miguel is singing to a former lover who suddenly wants to get back together. He starts by listing all the ways she let him down in the past. If only she had told the truth, answered his calls, and loved him back when he loved her, things would be different. But she didn't, so he hits her with the incredibly direct chorus: "Si no supiste amar / Ahora te puedes marchar" (If you didn't know how to love / Now you can leave).
He explains that he suffered a lot trying to forget her, and now that she's calling and swearing she's changed, it's just too late. He's moved on and found someone new, delivering the killer line, "Contigo yo perdí / Ya tengo con quién ganar" (With you I lost / I already have someone to win with). He knows exactly why she's back, admitting "que no hubo nadie que te diera lo que yo te di" (that there was no one who gave you what I gave you). But time has passed, and he's changed too. The message is clear: get lost!