Theatre allows you to write simple java classes that can be put onto the server in source code format, to then be compiled and loaded at startup/during reload.
Sometimes, you want to make a lot of different small adjustments for your server that don't fit into the same plugin. On the other hand, making a bunch of single class plugins isn't very time efficient either. The best solution would be to use Skript, but I can't be arsed to learn that and want to use the whole functionality of Java and the Bukkit API.
If you are in the same niche situation, then theatre is the solution.
If you want to compile kotlin with theatre, you need a directory at the root of the server called kotlin-compiler containing the kotlin compiler (https://github.com/JetBrains/kotlin/releases/).