A deblocking filter is a video filter applied to blocks in decoded video to improve visual quality and prediction performance by smoothing the sharp edges which can form between macroblocks when block coding techniques are used. The filter aims to improve the appearance of decoded pictures. It is a part of the specification for both Microsoft's VC-1 codec and the H.264 codec.
Read more about Deblocking Filter: H.264 Deblocking Filter, H.263 Annex J Deblocking Filter, Deblocking Filters As Post-processors