A media codec (coder-decoder) is software that compresses (reduces file size) and decompresses digital audio or video content for playback. Different codecs use distinct mathematical techniques, meaning a player application requires the specific codec used during the file's creation to understand and play it back. An "unsupported codec" error occurs when your player lacks the necessary codec software to decode that particular compressed data stream inside the media container file. The container format (like MP4 or MKV) is separate from the codecs used for the audio and video streams it holds.

This issue commonly happens when receiving files from others. For example, someone might send an AVI video file compressed with the older DivX codec that your modern media player lacks. Similarly, a web video encoded with the royalty-free VP9 video codec might fail in a browser or app built only for common codecs like H.264. Professional video editors also encounter this when working with camera footage recorded using specialized formats like Apple ProRes or AVCHD on systems missing those specific codecs.

A key limitation is that some codecs are proprietary, requiring licensing fees, which can deter universal software support. This fragmentation requires users to sometimes install codec packs or specialized players. While transcoding (converting) the file to a widely supported format solves the problem, it reduces quality and takes time. The industry trend towards free, open-source codecs like AV1 aims to improve compatibility and reduce such playback barriers in the future.