Flechó is the past tense of the verb flechar, which literally means 'to shoot with an arrow'. It's a highly romantic word that you'll almost always hear used metaphorically.
In the song, Jay Wheeler sings "Fue tu mirada la que me flechó" (It was your gaze that struck me). He's describing falling in love at first sight, as if he were hit by Cupid's arrow. This single word perfectly captures that magical, sudden feeling of being completely love-struck.
Have you ever sworn off love, only to have someone walk in and completely change your mind? That's exactly what Jay Wheeler is singing about in "Eazt". He starts by admitting, "Yo dije que no volvería a caer / Pero te miro y siento que caí" (I said I wouldn't fall again / But I look at you and I feel like I fell). It's a song about the surprise of falling in love when you least expect it, and falling hard.
He describes the feeling as being hit by Cupid's arrow, saying, "Fue tu mirada es la que me flechó" (It was your gaze that shot me with an arrow). This new love has completely transformed his world, pulling him out of a dark place and making him feel in perfect harmony with this person. He even uses a musical metaphor to describe their connection: "Mi melodía favorita es tu voz / Y sin darno' cuenta, estamos en el mismo tono" (My favorite melody is your voice / And without realizing it, we're in the same key). It's a sweet declaration of finding a love so perfect it feels like a song.