D&D Adventure Grid - Rules of Play says:

This sturdy double-sided, grid-lined play surface is ideal for when your Dungeons & Dragons campaign gets tactical. One side features a stone floor good for dungeon and city encounters alike, while the other features terrain useable for a wide range of rural and wilderness encounters. Additionally, the D&D Adventure Grid folds up for easy transport and storage, and can be used with both wet-erase and dry-erase pens interchangeably.