A voxel is like a pixel, but in three dimensions. Its a basic geometric unit that subdivides whatever object has been voxelized. In some contexts you may have a uniform voxel grid made up of boxes all of the same size, such as is the case for CT/MRI volume information. In other cases, the voxels may not all be the same size if they are purely for optimizing navigation through some geometry, in which case they may be very large voxels in regions that are not near material boundaries, with much smaller voxels near the boundary regions.