Not all kinds of terrain will completely block line of fire. Areas of terrain such as trees, shrubs, bushes and other similar plant-like obstructions will tend to form a 'grey area' where the enemy fades out of sight the further in they are.
The model terrain used to represent woods and jungles is made so that models can be moved into and through it. In reality woods and jungles would be overflowing with all kinds of greenery (or its alien equivalent) so in this case the usual rules for spotting targets using the model's eye view do not really work.
In woods and jungle it is possible for a model to see up to 6" into areas of woods, jungles or similar terrain. Models that are further than 6" into the terrain cannot be seen at all, or see out themselves. If both firer and target are within the same area of vegetation they can only see each other if they are within 6".
Note that although it is possible to see into an area of woods or jungle you cannot see through it even if it is less than 6" deep. So, if a unit is behind an area of woods or jungle they cannot be seen at all by anyone on the other side.
As a final note you may want to vary the distance that troops are visible in woods or jungles to their enemy. For example you could try reducing visibility to only 2-3" for very dense growth or increase it to 8-12" for sparse woodland.
Diagram: Line of Sight in Woods & Jungle — a winding jungle area. Space Marines standing 6" inside the jungle's edge can still be seen and targeted by Dark Eldar outside, since visibility into woods/jungle extends 6". Dark Eldar standing more than 6" inside a different, deeper patch of jungle have no line of sight out to Space Marines beyond it, since sight does not pass through woods/jungle, only up to 6" into it.