To the rest of the world Demeter is the greek god of the harvest, but here Demeter is a new crop properties api mod created by Wtoll to bring to fabric some of the same ideas as AgriCraft, and to rewrite the clunky crop properties system in my other mod Farmcraft. Demeter is licensed under the MIT license which for the most part means you can do whatever you want with it. If you experience any issues it would be much appreciated to report them to the issues tracker on the Demeter Github page.
It should be noted that out of the box Demeter only modifies vanilla minecraft crops. This is because end users must opt-in to having Demeter work for certain modded crops because of the different ways that crops can be implemented. You're probably sitting there really bummed because that sounds like a lot of work, but the official Demeter Extra Crops datapack is available here and contains compatibility with the vast majority of fabric's modded crops.
Demeter completely changes the way that you farm in Minecraft. Starting off there is a new properties system. Each seed has certain properties that make it more desirable to plant than another seed. For example, one germination of wheat seed might yield more upon harvest, while another might grow faster. At the moment there are two properties implemented, yield and growth. Yield gives players more seeds and "fruits" whenever they harvest a crop, and growth makes crops grow faster. Crops that are fully grown will randomly spread to adjacent unoccupied farmland, when that happens there is a small chance their genes will mutate and result in a seed with more desirable characteristics.