What is actor ownership?

Hi, everyone! Could anybody tell what is actor ownership? Why do we need it?

Ownership becomes relevant with Multiplayer and RPCs.

A client-owned actor cannot call RPCs from Actors that are server-owned.

What do you mean by that? PlayerController is owned by the client and can invoke Server functions. In fact, on clients, only client owned, replicated Actors can call RPCs.

Or did I misunderstand you?