Temblar means "to tremble" or "to shake." While it can refer to physical shaking, it often implies trembling from emotion, like fear, excitement, or nervousness.
In "17 Años," the word is used to describe the young girl's reaction to her first romantic embrace. She begins to temblar de miedo (tremble from fear), indicating a mix of apprehension and an unfamiliar, intense sensation, highlighting her innocence and the novelty of her first love.
Mexican cumbia legends Los Ángeles Azules kick this track off like a guy spilling secrets to his best pal at a party: "Amigo, sabes acabo de conocer / Una mujer que aún es una niña" ("Friend, you know, I just met a woman who is still a girl"). He explains right away that she is only seventeen, and he has already fallen head over heels for everything about her—flaws included. When he sings "Amo su inocencia, amo sus errores" ("I love her innocence, I love her mistakes"), he takes immense pride in being her very first boyfriend ("Soy su primer novio / Su primer amor").
The chorus hits with an intensely vivid scene of first-time romance. She is quiet and bashful ("Es callada, tímida, inocente tiene la mirada" [She is quiet, shy, she has an innocent look]), and physical contact completely rattles her world. Whenever he takes her hand, she feels something totally unfamiliar; when he pulls her into a hug, she literally shivers with nervous excitement ("empieza a temblar... de miedo" [she starts to tremble... from fear]). She has never felt anything like this before, leaving her wide-eyed, asking over and over: "¿Que si eso es el amor?" ("Is this what love is?"). It is an infectious dance track built entirely on that overwhelming, awkward buzz of discovering romance for the very first time.