Spectators and Game Masters are special roles that players can take. They influence what information players can see and how they interact with the game.
You can switch to being a spectator in the player options. Spectators don’t occupy a player slot and always have a grey cursor and chat color. In the player list, their color is transparent. Spectators cannot interact with objects on the table and other players can’t see their cursor. Voice and text chat work as for other players, though.
By default, spectators can’t see any hidden information: hand cards and held cards of all players are hidden, they can’t peek under cards and can’t use the stack or container explorer. There’s a setting in the session permissions that allows spectators to see all hidden information instead. When a spectator peeks or uses an explorer, other players are not notified like they would be if a regular player performs the same action. Another session permission allows you to disable spectators entirely.
In the session options under player slots, the host can assign slots to be game masters. A player in a game master slot can see all hidden information, like a spectator when the spectators see all permission is true. Game masters can also interact with anything: they can take cards from the hands of other players, for example.