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 the Computer and User collections, as well as fonts available for download. This collection appears in the Fonts window in an app.
Computer: Fonts installed in the Fonts folder in the system Library (/Library/Fonts/) and fonts available for download. This collection is shown only when the User collection contains fonts.
User: Fonts installed in the Fonts folder in your home Library (~/Library/Fonts/). To show your home Library folder, press and hold the Option key, then in the Finder choose Go > Library.