Clicking this allows you to use Google Fonts with the theme. When clicked a text field will be displayed where you can type the font you want to include.
Clicking the fonts.google.com will open a new internet browser tab to preview fonts.
Once you know the name of the font you want to use, start typing into the "Include Font" field and a list will start to auto-populate with matching font names. Continue to type out the name and click the font name in the list to select it.
Multiple fonts can be added by repeating the process above. Also to note, the selected fonts will preview using their font face.