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.
This song is a classic cumbia, and its story is told like a conversation between friends. The singer tells his buddy, "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's super excited about his new girlfriend, who is only seventeen years old. He seems to love everything about her youth, saying, "Amo su inocencia / Amo sus errores" (I love her innocence / I love her mistakes) and boasts that he's her "primer novio / Su primer amor" (first boyfriend / her first love).
The song then describes their interactions from his point of view. He says she's "callada, tímida, inocente" (quiet, shy, innocent). When they touch, she has a powerful and new reaction. He explains that when he hugs her, she "empieza a temblar / A temblar de miedo" (starts to tremble / to tremble with fear). This new, overwhelming feeling makes her ask if this is what love is: "Que si eso es el amor" (Is this what love is?). The song repeats this question over and over, leaving you with her sense of confusion and wonder at these intense new emotions.