The MKV file holds an unlimited number of video, audio, picture and subtitle tracks within the file. It is a convenient video file where you can fit in the highest amount of data possible. It is a RAW format that you can easily flip and turn all of the separate data tracks.
File saved into the MKV format more often than not save each track (subtitle, picture, audio and video) into a different dataline. This is similar to other container formats such as AVI or MP4 although generally when packaging an MKV file people rarely bundle all the tracks within the file. Most media players do not natively support MKV files, you would have to use a special media playing software to access its content normally such as VLC.