You were choosing a font in Gantry 5 right? The font you chose was a google font. The font-face definitions come from google's site. If you use Firebug (as I did in my screenshot) you will see the font definitions as provided from google. The font stack is something you write when you use the font in your CSS (or what get's provided as part of that font definition). e.g.
body {
font-family: "Open Sans", arial, sans-serif;
}
The above means load a font called "Open Sans" but if you can't find it or the font file is not right then use Arial font instead, if you can't find that then use any sans-serif font that you have available. So, it's a "stack" of fonts.
Regards, Mark.