Hey Unconnected,
Its called the 'fog of war' and im sure there would be a lot of resources out there to help you come up with the most efficient version if you are worried about performance.
Remember in the old warcraft games when you selected a unit you didn't have the same controls as the current RTS's . For example if you have a unit selected in starcraft 2 you can make him walk in a circle with enough clicks - try that in the old games and you would end up with a hexagon shape instead.
You have the right idea using arrays. See this 4 by 4 array below.
|0|0|0|0|
|0|0|0|0|
|0|1|0|0|
|0|0|0|0|
This array represents the overall map and the 0's are the spaces that are currently free to be moved onto. If your character is the number 1 then you can move any direction (this is how we determine collisions when they are made but the same thing can apply to what has been explored and what hasn't).
For the fog itself you have the option of using tilemaps or simply tiled background objects - depends on how your maps are set up really.