So, you've got your battlefield, but it's flat, featureless and devoid of life. While this might be appropriate if you're fighting over a starship landing field or deserted salt flats, it makes for a rather dull battle. Terrain is an important feature on all battlefields. A good commander will use the terrain to their best advantage, blocking the enemy's lines of fire, getting their heavy weapons into good firing positions and concealing their troops until they are ready to strike. Troops on the ground need cover and concealment if they are to survive against the deadly weaponry of the 41st millennium. A very large terrain piece can become the focal point of a battle.
If your budget is limited, simple pieces like rocks and logs can be represented by small stones and twigs scavenged from the garden, hills can be indicated with books placed under a blanket or sheet, and so on. Beyond this there are almost limitless opportunities for making scenery easily and very cheaply. With a little effort you will soon become an expert at building hills, jungles, craters, trenchworks, bunkers, temples and all manner of terrain. The list of different types of scenery is endless; from ash wastes to ice worlds, jungles to volcanoes, all could feature on your battlefields. White Dwarf magazine often contains articles about making scenery and is a great source of ideas and inspiration.