The first set of three nested loops is procedurally generating texmap, which is a multidimensional Array(16 * 16 * 3 * 16). 16x16 pixel textures, one for each the top, sides, and bottom, and then 16 of those. The inner loop tests against i, which is the current block "type" and performs customizations to the procedurally generated textures.

The next set of three nested loops is generating a random "world" with a tunnel cut out of it.

The renderMinecraft function is performing a minimal perspective projection http://en.wikipedia.org/wiki/3D_projection for a single ray cast into the world. Each pixel is cast and then, for each object hit by the ray cast, the closest is found, and a texture mapped pixel is calculated and written into the frame buffer.