This is just a dependency for Kotlin based mods. It doesn't do anything by it's own other than make Kotlin available for Kotlin compiled mods.
It's better to just use 1 file instead of bundling in the Kotlin runtime multiple times which both saves filesize of mods and issues with conflicts.
Just grab the current release from the Releases page.
Just unzip the Kotlin-Runtime into the Plugins folder of game or server.
You need to have gradle and java installed and then create gradle.properties file with contents:
PLUGINAPIJAR=C:/Path/To/PluginAPI.jar replace it with your actual location of the jar file on your system. Usually in either: C:\Program Files (x86)\Steam\steamapps\common\RisingWorld\Data\Java\PluginAPI.jar or C:\Program Files (x86)\Steam\steamapps\common\RisingWorldDedicatedServer\Data\Java\PluginAPI.jar