If you can't take the time to figure out what it does, then you're not the target market. It's really not that hard. It's a web framework for building static sites. As it says right in the hero of the site:
"Gatsby is a free and open source framework based on React that helps developers build blazing fast websites and apps"
"Gatsby is a free and open source framework based on React that helps developers build blazing fast websites and apps"