Renunciar means 'to renounce' or 'to give up'. It's a powerful verb that implies a formal and definitive rejection, much stronger than simply 'to deny'.
In this heartbreaking ballad, Selena sings, "Y aunque siempre lo renuncies..." (And although you always renounce it...). She uses this word to describe how her former lover completely disowns the love they shared, while she continues to see it as the most beautiful thing in her life. This dramatic word perfectly captures the pain of having your deepest feelings invalidated.
This song is a classic tale of unrequited love, but with a twist of self-respect. Selena sings from the perspective of someone who has just lost the person she loves. She accepts her fate, saying, "No me queda más / Que aguantar bien mi derrota y brindarte felicidad" (Nothing is left for me / But to endure my defeat and wish you happiness). She's heartbroken, but she's going to handle it with grace.
The really interesting part is how she deals with his denial. He claims what they had "no era amor" (wasn't love) and that it "nunca pasó" (never happened). Instead of getting angry, she holds onto her side of the story. She sings that even if she was "totalmente equivocada" (totally mistaken) about his feelings, she doesn't care. For her, it was real: "Por mi parte, lo más lindo / El más grande amor" (On my part, the most beautiful thing / The greatest love). It’s a powerful way of saying that her feelings are valid, even if they weren't returned.