Cilantro, the green leaf of the coriander plant. Nice and citrusy when chopped fresh, smells like a stinkbug when it’s stale. Some people seriously can´t stand the scent of it so it´s better to not mix it in food, just add it later.
Back to the wording of your question, “culantro” sounds like a nickname of the spice, probably from a cilantro-hater - or a kitchen worker -, as “culo” means a in Spanish.