Mosaic provides a selected subest of fonts from Google Fonts. Fonts are divided into two categories: Body fonts and Heading fonts
Body fonts have a high degree of legibility and can be used in any context.
Heading fonts generally contain more complex, decorative features and aren't approporiate for large blocks of text. Therefore, these fonts should only be used for small selections of text which use a relatively large font size, e.g. headings.
For more details on how to manage the fonts on your site, please see our Mosaic documentation on font management.