The Font Picker has three tabs: Choose Font, Google Fonts, and Theme Fonts. When you are editing a theme font, only the Choose Font and Google Fonts tabs are visible. When selecting a font on a widget, only the Choose Font and Theme Fonts tabs are visible.
The Choose Font tab lets you build a font definition, the Google Fonts tab lets you choose and optionally embed a google font, and the Theme Fonts tab lets you select one of the fonts defined in the theme.
The Google Fonts tab allows you pick one of the fonts offered by Google Fonts and either link the source of embed the font in the theme. We usually recommend embedding the font: it loads faster and your users will not require an open internet connection to download the font from Google.
The Theme Fonts tab is visible when selecting a font for the font property of a specific appearance.