The file extension for Matroska Video . It is a "container" format that can hold multiple video, audio, and subtitle tracks in a single file. Summary for the Viewer
This indicates that the file includes Polish audio or subtitles (Polski). Irreductible.2022.PL.BDRip.x264-K83.mkv
For those unfamiliar with "scene" naming conventions, here is what each part of that specific file tag represents: The file extension for Matroska Video