Le vent is “the wind”. Colloquially, it can also mean “empty words”, “hot air”.
Toutes ces promesses, c’est du vent.
Those are empty promises.
Un vent is also something different: a situation when someone ignores you or rejects you. It is usually used as mettre un vent (to reject someone) or se prendre un vent (to get rejected).
J’ai voulu lui dire bonjour, mais je me suis pris un vent.
I wanted to say hello to him, but he completely ignored me.
Elle a mis un vent au type qui la draguait.
She rejected the guy who was hitting on her.
Listen to the examples: