Observability
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
An environment is fully observable when the agent's stimuli reveal everything relevant about its current state, and partially observable when they do not.
Show
A chessboard hides nothing that matters; city driving hides the child behind the parked car.
Watch for
In fog, an agent must act on what it believes, not on what it sees — belief becomes the main event.