Post processing - the filtering techniques are applied to low-quality video to reduce distortion after the encoding process and improve visual quality. Post processing can make low-quality videos nicer to watch in a way that reduces blocking and ringing artifacts of the image.
Newest codecs (h.264, h.265, VP8/9, av1, h.266) include filters as part of the codec. That's why it's called “in-loop” filters. But even older codecs don’t have enough bitrate to make a nice appearance in the output video. It's full of blocking and ringing artifacts. Post processing can reduce it and increase the visual quality of the picture.