Note: This is a new option in GeneratePress 2.0
The option can be located in Customizer > General.
Many of your Customizer options write CSS for you, and output it to your pages. Instead of generating this dynamic CSS on every page load, we cache it in the database and serve the cached CSS instead. As you can imagine, this is a big performance boost.
On first load, the CSS will be cached and added to the database. Any time you save the Customizer options, the cache will bust and get updated with your new CSS.