Fascinas is the 'tú' (you) form of the verb fascinar, meaning "to fascinate" or "to captivate". It's a powerful word that goes beyond simply liking someone.
In the song, the singer says, "Pa' que sepas que tú me fascinas" (So that you know you fascinate me). He's expressing that he is completely enchanted and captivated by this person, almost as if under a spell. It’s a romantic and intense way to describe a powerful attraction.
This song is a super direct and sweet declaration of love. Los Primos del Este aren't playing games; the singer is completely crazy about a girl and wants her to know it. He tells her, "Mami, me tienes bien loco" (Mami, you have me so crazy) right from the start. It's a modern romance, too. He sees her online and admits, "Cuando subes fotitos al Insta / Siempre te mando corazones" (When you upload little photos to Insta / I always send you hearts).
He's totally captivated, saying it was love at first sight: "Sólo con verte mi corazón se enamoró" (Just by seeing you my heart fell in love). The whole song is about how beautiful he finds her, how much he wants to be with her day and night, and how he'd give anything for her. It's a simple, heartfelt message about being so into someone that they become your whole world.