They know the disposition of all tiles at all times. That means they know if a reousrce will be there as per the map gen. IOW when the map was generated, coal was placed on x tiles and that is known.
They do not know about resources that will be placed from being exhausted though, at least till they are placed.
It also means they know all of your units in towns or out. For the purposes of path finding, they have to behave as if they do not know tiles, beyond what they have explored. This is why I do not give out maps, till I have my land in my borders.
I do not want them sending settlers to open spots behind me. They know the spot is open, but cannot find the way, until they have exposed it by exploring or getting a map.