When we connect to a Game World with a Game Client, the traditional approach is that all Entities originating on our Client are session-based. This means that when the Client disconnects, they will disappear from the network World for all players.
A persistent object, however, will remain on the Replication Server even when the Client or Simulator that created or last simulated it, is gone.
This allows us to create a living world where player actions leave lasting effects.
In a virtual world, examples of persistent objects are:
A door anyone can open, close or lock
User-generated or user-configured objects left in the world to be found by others
Game progress objects (e.g. in PvE games)
Voice or video messages left by users
NPC's wandering around the world using an AI logic
Player characters on "auto pilot" that continue affecting the world when the player is offline
And many, many more
A persistent object with no Simulator is called an orphan. Orphans can be configured to be auto-adopted by Clients or Simulators on a FCFS basis.