Fulgor translates to "radiance" or "splendor." It's a beautiful and somewhat poetic word that evokes a sense of intense brightness and magnificence.
In the song, Julio Jaramillo uses it to describe the initial intensity and beauty of a love that ultimately proved to be false. He sings about "todo su fulgor" (all its radiance), highlighting the dazzling but ultimately fleeting nature of the relationship.
Julio Jaramillo's "Ayer Y Hoy" tells a classic tale of a love lost to betrayal and materialism. Our singer was deeply in love, believing he could make his partner happy, but it all turned out to be "en vano" [in vain]. He put all his hope into this relationship, only to realize that her promises were "juramentos falsos" [false oaths].
Now, he understands that everything was a lie. He was naive to believe her, thinking she would never deceive him. The pain is clear as he sings, "Ahora estaré solo para no sufrir" [Now I will be alone so as not to suffer]. The sting of betrayal is even sharper when he discovers she left him "por dinero" [for money], giving herself "a otro hombre" [to another man]. Despite the heartbreak, he believes she will eventually pay for her actions: "Algún día la tendrás tú, que pagar" [Someday you will have to pay for it].