"Fog of War" is a generic gaming term, describing the fact that the game does not allow you to view what is happening in certain parts of the game. In Europa Universalis II, this refers to the fact that - for certain certain provinces - you can only see who owns and controls them, and whether the province is currently under siege, or being looted.
It is not actually tied to the state of war at all, but is the case with provinces far away from your own, those of allies and countries you share royal marriages with, and territories where you do not have forces present.
When "fog of war" is in play, provinces on the main map will be darkened, and you will be unable to see any construction projects taking place in them, or any military forces stationed there.
In order to lift the "fog of war" on a land province, you must:
- own or control it,
- have a military unit in it or in a neighboring province, or
- have a positive diplomatic relationship (war coalition, royal marriage or vassalage) with its owner or controller.
For sea zones, you must:
- have a naval unit in it,
- after naval technology 18, have a naval unit in a neighboring sea zone, or
- after naval tech 18, own or control a coastal land province adjacent to it.