it's a fourth spatial dimension. if you imagine a cube, it has width, height, and depth. a 4d cube would have width, height, depth, and zhlerp, which is its size along a fourth spatial dimension. in three dimensions you can go forward and backward, left and right, up and down. in four dimensions you can go forward and backward, left and right, up and down, and zlep and wizz, moving along the fourth spatial dimension.