Once 2D FDCT has been carried out on 8x8 blocks of the image data, the coefficients are quantized. This involves use of a quantization matrix where we simply divide each FDCT coefficient by the corresponding value in the 8x8 quantization matrix.
The quantization matrix has bigger numbers on the bottom right and smaller numbers on the top left. Bottom right are the highest frequencies and conversely, top left is DC. Why is quantization not uniform?