A texture is a two-dimensional raster image applied onto faces or polygons for adding detail, surface texture, or color to a computer-generated graphical game or 3D model. Everything you see in a game is actually an image being rendered by the computer that is applied onto objects. Without textures games would take up far more resources and power than they can handle in real-time.
In zombie escape maps textures are often a very easy and efficient way of adding a lot of detail to a big environment while almost using no resources but the RAM, since most maps are very big and need to be completely optimized when combined with all the player and weapon models. However, textures do add towards the .bsp format of the maps, especially custom textures.