Flechó comes from the verb flechar, which literally means "to shoot with an arrow" (from flecha, arrow). You'll almost always hear it used romantically to describe being struck by Cupid's arrow.
In the song, Ana Mena sings "Debajo del árbol nos flechó cupido" (Under the tree, Cupid struck us with his arrow). It's a beautiful, vivid way to say they fell in love at first sight, making it a perfect word for a romantic Christmas tune.
Spanish pop star Ana Mena teams up with duo Jean & Alex for "Navidad Contigo", a cheerful holiday track that turns Christmas into pure romance. The whole vibe centers on reuniting after a long time apart—specifically, "ha pasado un año y hoy por fin te vuelvo a ver" ("a year has passed and today I finally see you again"). Instead of just focusing on typical holiday traditions, they treat the season like a winter date. They play out in the snow, warm up when it gets freezing, and share kisses under the tree ("en el árbol tú y yo nos besamos"). In fact, the romantic spark is so intense that they flat out call it "como un San Valentín" ("like a Valentine's Day").
There are a couple of fun cultural lines to listen for while learning the lyrics. One singer jokes that Cupid struck them right under the Christmas tree, and calls their partner "mi estrella de Belén" ("my Star of Bethlehem"), the bright biblical guide used here as the ultimate compliment. You will also catch the Puerto Rican street slang "Santa se guilló conmigo" (which loosely means "Santa hooked me up big time" or "Santa outdid himself for me") by granting "una Nochebuena más contigo" ("one more Christmas Eve with you"). It is a sweet, upbeat reminder that the best gift under the tree is waking up next to the person you missed all year.