Scenery is a self-explanatory term that is used to describe almost any object in the environment which is not part of the BSP(the model used for the level). Majority of these can be placed in Forge(in Halo 3), although some can only be placed in Sapien, the tool used by Bungie in every game in the Halo trilogy(including ODST) to populate the levels. These became movable since Halo 2 and every game after that.