MKV is a container format for holding multiple audio, video, picture, and text data such as subtitle tracks. It is similar to other common containers like MP4 and AVI, although MKV is open in the specification. For video contents, MKV uses a .mkv filename extension, .mka for audios, and .mks for subtitles.
MPC-HC, KMPlayer VLC Media Player MKV File Player MPV
HEVC (High Efficiency Video Coding), is a modern video compression standard developed as the successor to the standard H.264 (Advanced Video Coding). In comparison to H.264, this format offers up to 50% better data compression while providing improved video quality at a similar bitrate. Today, HEVC is used by over 40% of video developers, only second to AVC.