Terrain is the area where you play your tabletop game, representing the environment where the characters are located. In a wargame like Warhammer, this could be a large battlefield, in Dungeons & Dragons this could be an elaborate dungeon or castle. Really anything is a possibility.
Some people use simple grid maps with 1-inch squares, providing a quick simple reference for scale. Obstacles and points of interest can just be written in with a pencil or erasable marker. This is certainly the cheapest and easier method.
When simple maps are cutting it for you, you can start adding rocks, trees, doors, whatever works. Some people go for realistic, detailed pieces that would fit in a high-quality dollhouse. Currently, I’ve been making terrain out of 2-foot diameter discs, and then using interchangeable scatter terrain to customize the scene as needed. My zombie parking lot was made using this technique:
And here’s a forest I’m working on:
Next will be some dungeons/caves, for classic underground exploring. The nice thing about this option is that it’s customizable, and I can focus on the elements that are most important to the scene. I don’t need to put in every wall or crack, but there’s enough pieces to add interest and interactivity. Plus I can just move them around for the next battle/scene/game. And as an added bonus, I put the disc on a lazy-susan-style turntable so I can rotate it and every person at the table can see every angle.
Of course, some people go ALL IN, and build stuff like this.
I have neither the time to build this nor the space to house it, but I absolutely salute and envy the folks who have the skill and patience for this kind of craft.