Codec (short for compressor/decompressor) is a compression format used in video, such as MPEG-4 or DivX.. Your computer media player (for example, Windows Media Player, QuickTime or iTunes) should recognize the codec required for a particular video, and play the video automatically. If it does not, you will need to find and download the correct codec yourself. Here are some resources to help you find one: Video Help or Movie Codec.