Necio translates to "stubborn", "foolish", or "obstinate". It's a powerful word that goes beyond simply being wrong; it implies a stubborn refusal to see reason.
In this bachata classic, Romeo Santos uses necio—the song's title—to describe his state of mind. He knows it's irrational to pursue a woman who is already taken, but his overwhelming feelings make him act like a necio, a fool in love who won't give up.
Romeo Santos is being a total fool for love in this song, and he knows it. "Necio" means stubborn or foolish, and he's singing about being completely obsessed with someone who is already in a relationship. He feels like a little devil on his shoulder is pushing him to do things he shouldn't, like sending flowers and calling her at night, even though she is "ajena" [someone else's]. He just can't accept her as only a friend.
This hopeless crush is making him act wild. He calls himself "un loco hablando con reflejos que no están" [a crazy man talking to reflections that aren't there]. She is his "fruta prohibida" [forbidden fruit], and the desire to have her is overwhelming. He admits the feeling is irrational and is consuming him, saying his stubborn love has thrown itself into the ocean and now, "me estoy hundiendo" [I'm sinking].