Tembló is the past tense of the verb temblar, which means "to tremble" or "to shake". It's a powerful word that creates a very physical image of a strong emotion.
In the song, the singer describes the moment he fell in love by saying, "Todo tembló dentro de mí" (Everything trembled inside of me). He uses this metaphor of an internal earthquake to express the overwhelming, full-body shock and excitement of love at first sight. It's a much more dramatic way to say you have butterflies!
Have you ever looked at someone and immediately realized you were done for? That is the exact story Mexican pop-rock group Camila tells here. The narrator describes love at first sight as an instant visual shift: "De blanco y negro a color me convertí" ("I turned from black and white to color"). Everything shakes inside ("Todo tembló dentro de mí"), and he claims destiny arranged it: "El universo escribió que fueras para mí" ("The universe wrote that you were for me"). It is big, dramatic, and completely honest about losing control with just a single glance ("Fue entregarte mi amor con una mirada").
What makes this track fun is the slight panic underneath the romance. He admits dropping the L-word this fast sounds wild: "Sé que no es fácil decir 'te amo', yo tampoco lo esperaba" ("I know it's not easy to say 'I love you', I didn't expect it either"). Yet, he rushes to confess before any more time slips away, calling this person the love of his life ("eres el amor de mi vida"). It is an overwhelming rush of feelings that makes zero logical sense, but as he puts it, there is no way to explain it and even less reason to doubt it ("no hay cómo explicar pero menos dudar").