It's a tabletop role playing game - like a solarpunk scifi version of DnD (or the TTRPG of your pick). The rulebook and other resources available there enable people to play it. Specifically it helps one individual (the GM) run their own campaigns/games, by giving them a suggested set of rules and a vibrant setting they can use all or some of (or just take inspiration from), and it helps the rest of a group of players to create characters and interact with the setting. Together they do a sort of collective storytelling.

In the broader scope of what it does, hopefully it helps people who aren't super familiar with solarpunk and it's associated philosophies and movements to imagine a better world, other ways we could do things as a society.