if (vtest[z][x][y] > 0
&& (vtest[z][x + 1][y] > 0 && vtest[z][x - 1][y] > 0 && vtest[z][x][y + 1] > 0 && vtest[z][x][y - 1] > 0 && vtest[z + 1][x][y] > 0 && vtest[z - 1][x][y] > 0))


this just means:
if the voxel is not 0 (air) and all voxels around it are not air,
then the voxel can not be visible. (simply because there
is not way to see it, as its sourrounded by solid blocks)

the rest is just for looping though the x,y,z array