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 on the left in Font Book:
All Fonts: Every font associated with the Computer, User, and Network collections, as well as additional system 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 additional system fonts available for download. This collection is shown only when the User and Network collections contain 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.
Network: Fonts installed in the Fonts folder in the Library folder on a network computer (/Network/Library/Fonts/). This collection is shown only when you’re connected to a network computer that has fonts installed in that folder.