de a de veras is a colloquial Mexican expression that means "truly" or "for real." It's an emphatic version of the more common phrase "de veras" (really).
In the song, Xavi sings "Te quiero, yo te quiero de a de veras" to express that his love is absolutely genuine and sincere, not just a fleeting feeling. Learning this phrase is a great way to add authentic, heartfelt emphasis to your Spanish.
Picture this: people are talking trash about you behind your back, telling the person you like that you are bad news. That is the exact mess Xavi walks into here. Instead of getting angry, he picks up his guitar and tries to set the record straight with pure, dizzy romanticism. He admits people are gossiping with "Hay rumores, yo lo sé" [There are rumors, I know it], and warns that folks tell her "Que te puedo hacer sufrir" [That I can make you suffer]. His defense? He is completely head-over-heels, swears he is faithful, and promises nobody else is occupying his thoughts.
He doubles down on how wild it is that they even met out of the entire world: "Entre miles de millones nos topamos" [Among billions we ran into each other], calling that pure fate and giving credit to God. When he sings "yo te quiero de a de veras" [I love you for real], he uses the everyday phrase de a de veras, which is a folksy, conversational way to say something is genuine rather than make-believe. He readily confesses "Estoy loco, no lo niego" [I am crazy, I don't deny it], which makes the plea feel dramatic and a little desperate in a charming way. When talking doesn't clear your name, writing a song to hand over your whole heart is pretty much the ultimate move.