Arrepentirás comes from the verb arrepentirse, meaning "to regret" or "to repent." It's a strong emotional verb that conveys deep remorse or a change of heart.
In "Quédate," the singer uses it in the phrase "no te arrepentirás" (you will not regret it) to desperately convince his love to stay. He's promising that if she remains with him, she won't experience any regret, highlighting the intensity of his plea and the emotional stakes of their relationship.
Alzate's "Quédate" is a passionate plea from someone on the brink of losing the person they love. He feels his partner pulling away, saying goodbye with words, but he sees her tears and knows it's not true: "tus lágrimas confirman que es mentiras, tú aun me amas, estás confundida" [your tears confirm it's a lie, you still love me, you're confused]. He believes he taught her everything she knows about love, and the thought of losing her is incredibly painful.
He's desperate for her to stay, promising she won't regret it: "Quédate conmigo que no te arrepentirás" [Stay with me, you won't regret it]. He declares his unending love, saying he loves her more than anyone in the world and always will. Even if there's someone else, he wants her to be honest, but again, her tears tell him she's still in love with him. It's a powerful song about fighting for a love he believes is still there.