It is used to ask or talk about the person someone is doing something with.
Here are some examples:
* ¿Con quién vas al cine? (Who are you going to the cinema with?)
* Estaba hablando con quien me llamó. (I was talking with whoever called me.)
* Viajé con quien me invitó. (I traveled with whoever invited me.)
It's important to note that "con quien" is a two-word phrase. "Con" means "with" and "quien" means "who" or "whom."