Press F2 to enter Map Editor Mode. Once in the F2 menu you will see the Terrain tab. For ease, the Terrain tab is seen highlighted on the right in the below screenshot with the terrain options appearing below.

Below is a short video that shows you how to use the no_pass and no_pass_human terrain options found within the Terrain tab in F2 mode. Using this feature allows the map designer to prevent access to areas of the map. It can be used for vehicles (no_pass) or soldiers (no_pass_human_ or both. I sometime use this feature to prevent vehicles taking an inappropriate path and getting stuck.

The below video is an example of how I use it.

As you will see in the below video, the Swamp terrain gives the effect of your men drowning in quicksand. Be mindful of where you place this on a map, as those who play your map may not appreciate losing their soldiers, although it will cause you to have an evil laugh at their expense.

And this is what happens when vehicles enter the swamp terrain. The vehicle has three occupants out for a Sunday drive.

In the below video I show you how to apply different Terrain FX found in the Terrain tab of the Map interface.
Terrain FX are as they state, effects that can be applied to the terrain that provide some sort of realism to the graphical experience. Think water ripples over puddles, dust on a country dirt road. Snow being thrown from tank tracks etc. You get the idea. Now for the video.

This chapter shows you how to use the Texture Fill option in the Terrain Menu. This is particularly effective for large maps. It makes the job of applying terrain much quicker.
At times you may need to go in and fine tune certain areas where the Texture Fill process has been confused by the textures you have chosen.