Vision is a hidden stat that changes how many tiles are uncovered whenever you move. This can be increased by traits such as Magic Vision and Heightened Sense (multiples of these traits do not stack, your Vision will only increase by 2 if you have both of these on the same character).
Party Vision is equal to the highest character's Vision stat, this means that characters who have the Short Sighted trait are almost never a hindrance since they won't affect the actual vision stat. All of your characters would need to have this trait for it to actually affect your Vision negatively.
The Glowing trait removes the Vision penalty of almost every single tile, excluding the Lake tile.
The default amount of tiles that will be uncovered depend on the type of tile that you move to, for example, if you move to a Deep Forest tile which has -3 Vision while you only have 2 Vision then you will only be able to see 1 new tile in each direction (since minimum vision is always 1), but if you move to a Mountain tile which has +2 Vision while you have 3 Vision then you'll discover 5 new tiles in each direction.
Note that Portals with the Dark suffix decrease your Vision by 1 and the Bright suffix increase your Vision by 1 whilst inside.
The Vision value for each tile are as follows: