A media container packages video, audio, subtitles and related information into a file or delivery structure. It describes how those components are organised rather than prescribing one universal codec. A player must understand the container and the encodings inside it, so a familiar extension does not guarantee that every contained track can be played.
A practical example
Two MP4 files can use different video and audio encodings. A device may play one correctly and struggle with the other, even though both filenames end the same way and both files open in the player's browsing interface.
A common misunderstanding
Changing a file extension does not change its actual container or convert its media tracks. Identify the real structure and codec requirements before treating a rename as a solution.
Related terms
Further reading
Stable technical definition. Editorial approach