On first launch, FontBase will only have the pre-loaded Google fonts. When you add any other fonts to the app, by default, they are copied to the FontBase Root folder, which is located at /Users/USERNAME/FontBase on Mac, and C:/Users/USERNAME/FontBase on Windows by default. So, if you want to add any other fonts to the app, you can simply copy them to this folder. Also check out the Adding Fonts page for more details.
Right away you can search for fonts, explore the Family page, organize your fonts into collections, apply different styles and much more. When you need to use some fonts in other apps, simply activate them by clicking the grey circle near a family title. When the circle turns green, the fonts are ready to use.