They are used to create new biomes, resources and structures within the game.
Here's a simple guide for how to get it installed, and configured ready to play! The install is easy through steam UI, and setup and configuration is as simple as editing a text file.Chest= 10x leather, 16s craft, 1 item lvl gain per craft, 7x leather scrap. Total cost = 16 seconds and 4 leather for 1 item lvl point gain. Boot = 5x leather, 8s craft, 0.5 item lvl gain per craft, 3x leather scrap return. Leatherworking: Working on it right now and just did the math. Quest automatically repeats after completion for additional skill points. Rewards player with +1 skill point for every 100 blood moon zombie killed. This quest is automatically given to the player after the "Basic Survival 8/8" quest is completed. Addition of new "Blood Moon Vanguard" quest.
If put on a Multiplayer server (unzip into /Mods folder), it will be available to all clients/players. It can be put on single player (just unzip into /Mods folder). The modlet is a small addition with a Wireless Powerbox, configured to hide outgoing cables for electricity.