Terrain is made up of two components: terrain features and terrain areas.
Terrain features are individual pieces of scenery, such as ruins, barricades, trees, rocks, and anything else you could conceivably encounter on the battlefields of the 41st millennium. Terrain features will always be placed on (or sometimes used to denote the edge of) a terrain area. Several key changes now allow for a wider variety of terrain features on the battlefield, and a greater degree of interaction with terrain than before. A terrain area is the footprint on the battlefield under the effects of that scenery, and these effects are undergoing some big changes compared with the current edition.
Foremost amongst these changes is the addition of the Hidden rule. Infantry, Beast, and Swarm models inside a terrain area can be Hidden as long as their unit didn’t shoot in the current or preceding player turn (and so will be hidden at the start of the game). While hidden, a model is only visible to enemy units within their detection range – which is usually 15” – so you can deploy your models a lot more freely without worrying about a first-turn barrage.
Additionally, most terrain is Obscuring, much as it is today. Obscuring terrain areas cannot be seen entirely through, so even large models and big units can use sections of terrain to avoid enemy fire. Infantry, Beasts and Swarm units in terrain areas also gain the benefit of cover, which in the new edition gives your opponent a -1 penalty to their Ballistic Skill, rather than the +1 bonus to armour saves you receive now.