Fog or 'Fog of War' is a game setting which is used to make games more realistic by simulating the uncertainty in situational awareness experienced by combatants in a real war. There are varying levels of fog which at the board designers discretion can be overriden by the game host when the game is created. The fog setting for the game is displayed on the game join page so you can check this before joining the game.
For the heavier fog settings the action of attacking a neighboring territory can reveal additional information to you - for example, when playing with heavy fog, attacking a territory will show who is currently occupying that territory and how many units they have placed there.
Note that when playing a team game there is a 'Team Vision' rule which if enabled allows the player to see the territories and unit numbers that their team mates can see also.
Screenshots are that of the Green Player.
None Player can see the occupying player and the unit count for all territories on the board.
Light Player can see the occupying player for all territories on the board but the unit count for bordering territories only.
Medium Player can see the occupying player and unit count for bordering territories only.
Heavy Player can see the occupying player but not the unit count for bordering territories only.
Total Player can only see their own territories.