Google Fonts is Google's catalog of fonts for websites and apps. All of its fonts are released under open source licenses, which allow use in personal and commercial projects. The font files and their metadata are kept in a public GitHub repository, which is also the issue tracker for the project.
A web page uses a font through the Google Fonts API, which needs no programming: the page adds a stylesheet link to fonts.googleapis.com with the font family name, then names the font in a CSS rule (CSS is the language that sets how a page looks). Parameters in the link select styles and weights, script subsets and the font-display loading behavior, and the text= parameter loads only the characters that a page needs. A Developer API returns the list of families that are currently available. Because every font may be redistributed under its license, sites can also host the files on their own servers.
The pages state no price: the fonts are open source, and each font family folder in the repository holds its license file.