Juro is the first-person form of the verb jurar, meaning 'to swear' or 'to take an oath'. It's a powerful and dramatic word that instantly adds weight and sincerity to a promise.
In the lyrics, the singer declares, "Te juro ante Dios / Nunca te voy a faltar" (I swear to you before God / I will never fail you). Using juro transforms a simple promise into a solemn, unbreakable vow, showing the absolute depth of his commitment.
This song is a massive, heartfelt promise of total devotion. NSYNC is singing to someone who is going through a tough time, maybe after a bad breakup. He sees their sadness and the "vacío / Que no puedas llenar" [emptiness that you can't fill] and steps in to offer complete support.
The promises are huge and very dramatic. He says he'll make them forget what made them suffer and that they won't fall as long as he's there. He's not just offering a shoulder to cry on; he's offering to be their everything. He swears, "Si siente un frío tu corazón / Seré tu abrigo, tu ilusión" [If your heart feels cold / I will be your coat, your illusion]. The commitment is for life, repeating the vow, "Hasta ya no respirar / Yo te voy a amar" [Until I can no longer breathe / I'm going to love you]. It's the ultimate pop ballad declaration of love.