If you are using GED to paint the textures onto your terrain, you don't need to worry about what Spirit said. GED does all of this for you automatically.

Alternatively, what Spirit is talking about, you can create your own textures that match the size of the terrain, and you use the alpha channel to show where you want your RGB texture to be tiled. (White in the alpha is where the texture will show, black is where it will not.) The texture must be the size of the terrain because the alpha channel of the texture will cover it 1 to 1 in order to show where you want your tiled RGB texture to be placed. You then add the textures to the terrain in MED in order from first (lowest) layer to last (highest layer). You can use an unlimited number of textures but the more you use, the slower it becomes. I am currently using just three. Assign the multi texture terrain shader and that's it. You can also adjust the amount/size of the texture tiling with detail_size.

Below are some images that might help out.