A film in MKV can carry several soft subtitle tracks inside the file. Extracting a track gives you the subtitles as a separate SRT file to edit, translate, resync or use with another copy of the film. Tracks that are pictures, as on DVDs and Blu-rays, cannot be extracted as text.
One MKV can produce several SRT files; when it does, they are offered together as a ZIP download. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because SRT cannot be written by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB. Only text subtitle tracks stored inside the video are extracted, one SRT file per track (several tracks come as a ZIP); picture-based tracks from DVDs and Blu-rays are refused because they would need text recognition. A feature-length file is fine, since the video is only read and the result is a few kilobytes.