From the Core Rules
FAQ page 1.
OBSCURING AND DENSE COVER
Obscuring and Dense Cover are two terrain traits introduced with ninth edition that interact with visibility. These rules do not overwrite the normal rules for determining visibility, though – they are in addition to them. Specifically, even though the Obscuring rules state that Aircraft and models with a Wounds characteristic of 18+ can be seen through Obscuring terrain, they are still only visible (and hence eligible) targets if the firing model can physically see them (so if the terrain in question is solid and opaque, they are still not eligible targets). Also, in the same way that Obscuring terrain ‘blocks’ visibility when it is in between the firing model and its intended target, Dense Cover terrain imposes a hit penalty whenever it is between the firing model and its intended target (with the noted exceptions). It is not required for a unit to be fulfilling the criteria of ‘gaining the benefits of cover’, as described for Obstacles and Area Terrain, for this penalty to hit rolls to apply (but also note that any rule that ignores the benefits of cover, or that ignores the benefits of cover that impose a penalty on hit rolls, would still ignore that penalty).