Fontsource is an open-source project that packages open-source fonts into individual npm packages, so that a web application can host the font files itself instead of loading them from another company's servers. npm is the package registry of the JavaScript ecosystem. The site lists 2,103 font families, from the Google Fonts collection and from outside it.
A developer installs a font package and imports it in the app's code. The package holds the font files and the CSS @font-face rules, so the fonts ship with the app, stay at a fixed version like any other dependency, and load offline. Self-hosting also removes the extra DNS lookup and TCP connection that a third-party font CDN needs, and avoids Google's tracking of font use. For cases that need a stable link, Fontsource also runs a CDN (content delivery network) through jsDelivr that serves versioned CSS stylesheets and font files by URL.
The project's code is under the MIT license and free to use. Each font keeps its own license; most use the SIL Open Font License 1.1.