If you aren't using VLC, you may need the K-Lite Codec Pack to provide your system with the necessary decoders.
If the filename was generated by a ripping tool and you need to know how to use it: If you aren't using VLC, you may need
MKV is a "container" format that can hold multiple video, audio, and subtitle tracks. Not all default players support it well. If you aren't using VLC
If you need to change subtitles, remove audio tracks, or fix metadata: remove audio tracks