What is KubeJS?

KubeJS is a mod that creates JavaScript bindings for Minecraft that allow you to manage your server, change recipes and edit functionality.

The mod splits this functionality into startup_scripts, server_scripts and client_scripts.

startup_scripts manage changes to world generation or registering new items and blocks or integrating directly with Forge events.
server_scripts handle both adding and removing recipes, changing tags and modifying loot tables. They can also handle server events like a player right-clicking a block.
client_scripts are loaded every time client resources reload, they are used for interacting with clientside mods like JEI or REI and adding tooltips to items.

Further reading