MKV got its name from the Russian matryoshka which is the way the nesting dolls are called. MKV functions as a container format (as opposed to a codec).
It can consist of several items within a single file, and they can range from video, audio, images and/or subtitles.
This file format gained popularity among those who create subtitles and want to integrate them within videos.
MKV makers will allow users to create such files by selecting all the containing items, including subtitles.
Before choosing the output filename for the generated MKV video, users might also modify some video parameters.