Kotlin is a JVM flavor by Jetbrains.
Note: I do not own nor develop Kotlin; this is just a plugin that includes Kotlin in it for convenience or shared use.
Kotlin is licensed under the Apache 2 license.
This plugin adds the Kotlin libraries to the server, making plugins written in Kotlin usable by the server.
If you have written a plugin in the Kotlin language and it is unusable because of compatibility problems or missing libraries, use this plugin to use Kotlin as plugin language.
With this plugin on your server, you are no longer forced to keep fiddling around with the Gradle or Maven settings to add Kotlin support for your plugin.
Just install this plugin on your server like a normal Bukkit plugin, and put your own plugin, which may be partly or fully based on Kotlin, in the "plugins" folder of your server just like a normal Bukkit plugin
In general, this plugin adds all Kotlin libraries, so all Kotlin Bukkit plugins on the same Bukkit/Spigot/Paper server will be compatible