What describes the Terrain DetailLayer?

Hello everyone, I created a Terrain and want to populate it with grass. So I created a GrassPrototype and added it to the Terrain as DetailLayer. I mostly did it like in the Documentation but there is somet$$anonymous$$ng I don't understand:

In the code it is written:

   // For each pixel in the detail map...
   for (var y = 0; y < t.terrainData.detailHeight; y++) {
     for (var x = 0; x < t.terrainData.detailWidth; x++) {
       // If the pixel value is below the threshold then set it to zero.
       if (map[x, y] < threshold) {
     map[x, y] = XXX;
       }
     }
   }

But what exactly does the map[x,y] XXX value do? I noticed that a value of 1 seems to spread it evenly throughout the terrain w$$anonymous$$le a $$anonymous$$gher value seems to group it to certain places. Can anyone please explain that? Thank you very much!

Regards, Tobias