Mythic Battles: Pantheon is played on different boards each divided into areas. Each of these areas has a terrain type, indicated by an icon. If the ground is empty of 3D terrain, then no rule applies except in special cases (stairs, water, …). If you place a 3D terrain element on an area, the terrain type is then activated, and a small special rule applies.
For example, if you place a tree on a forest zone, the forest rule applies: units in the forest gain a +1 bonus against ranged attacks. If you place a column on a ruin area, then the units in these ruins benefit from the ruin rules and thus gain a +1 bonus against melee attacks. Ruins and forests are examples of the game scenery, and are found on the various boards available in the core box and the expansions.