Font collections
A font collection is a group of fonts, such as fonts installed in the same location or fonts used for websites. In Font Book, you can use collections to organize the fonts on your Mac. There are also default collections, which are listed in the sidebar in Font Book:
All Fonts: Every font associated with system and user collections, as well as fonts available for download. This collection appears in the Fonts window in an app.
My Fonts: Fonts that you installed. This includes fonts in the Fonts folder in your home library (~/Library/Fonts/), /Library/Fonts, and any user library. To show your home Library folder, press and hold the Option key, then in the Finder choose Go > Library.