zstd is a compression program.
The zstd kioslave is not directly usable, and is intended for use
as a filter. For example, the tar kioslave can filter a file through
the zstd kioslave, in order to display the contents of a
tar.zst file directly in a Konqueror
If you click on a file compressed with a
zst extension in Konqueror, this kioslave is
used to uncompress it and display it as a normal (uncompressed)
If you are a developer, and would like to use the zstd filter, you can find documentation on using kioslaves at http://techbase.kde.org
See the manual: zstd.