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It's in the Game Guide:

Quote:

New Full (CD) Game only Features:

We have expanded the information displayed when you pass the mouse cursor over a hex for the CD version of the game. Previously, the only information about a hex was Height and the terrain types in it. The new display includes ground height, obstacle height , total height and Terrain Density displayed like this :

Ground Height / Obstacle Height / (Combined Height) Density XX
And might look something like this in the game for trees

Height 1/18/(19) Density 26
Ground Height and Obstacle Height are self explanatory but Terrain Density , while being an old concept for the game, it will be a new one to most players. Terrain Density or just “Density” for short is a value the game uses to determine how solid or transparent a bit of terrain is. For example, trees block Line of Sight (LOS) to the next hex when they are greater than >30 density. If they are under 31 LOS is deemed not fully blocked and you can see into the hexes beyond that hex.

Density is cumulative for purposes of LOS. A hex with trees that have a density rating of 20 will not block LOS to the hex beyond it but if that hex has trees that are also have a density rating of 20, LOS will be blocked beyond that hex. Theoretically you could have a number of sparsely treed hexes in a row before LOS would be blocked but typically in the game the maximum would be two but please note those two hexes do NOT have to be adjacent. The game has worked this way since SP1 was first released, we are just displaying to players for the first time.
While Ground Height and Obstacle Height might be self explanatory, how the game utilizes them together with Density requires some explanation.

Ground terrain ALWAYS blocks LOS 100% and it blocks LOS when it is more than 3 “units” of height above the level your unit is on. For example, if your unit is standing on a hex that is base ground level ( i.e. zero elevation ) that units LOS will be blocked by ground that is 4 units or greater high.
Obstacle Height combined with density also blocks LOS and once again it is cumulative. For this I will use tall grass as a general example. Tall grass is 2 units high and has an average density in the game of 10. If the ground the grass is on plus the grass itself is greater than a total of 4 units high LOS will be blocked when enough density is accumulated to do so. In some cases this might take quite a number of hexes to achieve if the terrain between each >3 units of total height hexes are <4 units of height high
Each "unit" of height in the game is now considered to be roughly 24 inches ( 61 cm ) so each 10 unit level of elevation in the game represents roughly 20 feet ( 6 meters )