Traditionally a Minecraft Server is a Java application, being run inside the terminal. Due to this, opening a server to play with friends can be compicated or expensice for new users.
Cauldron.js instead moves all computing and disk space intensive work into the browser, while keeping all communication with the Minecraft clients on a centralised server.
This allows possibly hundreds of Minecraft Servers to be run on a single, inexpensive server while players can simply open a webpage to start a Minecraft server.
Besides learning those technologies, I also learned about optimizing the server-client connection in order to make the Minecraft server perform as best as possible.