So, I actually didn't find any official tutorials on this topic, so here's a little rough explanation by me.
In order to avoid the repetitiveness you have a selection of different textures you can apply to the terrain.
The diffuse texture provides color for the terrain, you should use it for the over-all color and not for finer details. If you only have a diffuse texture, then it's okay if it looks blurry, it just shouldn't look repetitive and it should reflect the color of the terrain at this area. For example, green for grass or white for snow.
The detail texture is used for the up-close texture of the terrain. This is usually a gray-scale image, or even better a high-pass filtered image. It applies a detail to the underlying diffuse color, for example pebbles for rocks or straws for grass. It can contain colour, but often the best results come with less color in the detail texture, as you can then have different nuances to your detailed texture based on the diffuse texture.
The macro texture, afaik, has the same role as the detail-texture but is used for medium-distance detail instead of the close-up.
Lastly, the normal texture contains normal and height-map information for even cooler detail.
What I often do, is to set the diffuse texture to a huge texture which is as big as the terrain-heightmap (e.g. 2048px * 2048px). This allows me to paint the overall terrain-color anyway I want and with no repetition. E.g.
Then I can control the micro-detail with the detail and macro maps.