Flechará is the future tense of the verb flechar, which literally means 'to shoot with an arrow' (from the noun flecha, or 'arrow').
In Spanish, this verb is almost exclusively used in a romantic context to mean 'to make someone fall in love,' as if they've been struck by Cupid's arrow. The song uses it poetically, "Cupido los flechará" (Cupid will strike them), to describe the instant, magical connection that happens with just a kiss.
Ever had a kiss so good it felt like a whole relationship? That's the idea behind this Aventura classic. The singer tells us about a woman who "domina mis sentidos con sólo tocar mi piel" (controls my senses just by touching my skin). He falls completely in love from a single kiss, even admitting "ni siquiera la conozco bien" (I don't even know her well). It's a wild idea, but he insists this isn't just about him—it could happen to any man.
He then gets philosophical about it, saying a kiss means "amistad, sexo y amor" (friendship, sex, and love) anywhere in the world. For him, this one kiss is almost a religious experience. He says "Por un beso de su boca voy al cielo y hablo con Dios" (For a kiss from her mouth, I go to heaven and speak with God). It's an intense and passionate take on the power of that first connection.