Please help me with terrain generation!

[dangerdoc]

I have asked this before, and I have gotten answers. I could not learn what I needed from those answers. I need an explanation of the process that is involved in 3-D trillinear terrain generation. I have searched for a long time but have not found something that I can understand. I think visually, and I need an explanation about what this (example) means:

value += fractX       * fractY       * fractZ       * noise[x1][y1][z1];
   value += fractX       * (1 - fractY) * fractZ       * noise[x1][y2][z1];
   value += (1 - fractX) * fractY       * fractZ       * noise[x2][y1][z1];
   value += (1 - fractX) * (1 - fractY) * fractZ       * noise[x2][y2][z1];

   value += fractX       * fractY       * (1 - fractZ) * noise[x1][y1][z2];
   value += fractX       * (1 - fractY) * (1 - fractZ) * noise[x1][y2][z2];
   value += (1 - fractX) * fractY       * (1 - fractZ) * noise[x2][y1][z2];
   value += (1 - fractX) * (1 - fractY) * (1 - fractZ) * noise[x2][y2][z2];

means. I cant make sense of code like that without an explanation. How do you process the information, and what is the reason for each line of code? If I learn to do 3-D trillinear interpolation, I am willing to make a tutorial that explains it to others like me. Thank you for reading this post, and thank you even more if you are willing to walk me through this.

dangerdoc