A font stack is a series of fonts - as the site loads the browser will descend down the font stack/list till it finds a font it has access to.

e.g. Garamond, ‘Times New Roman’, Times, serif;

which in css would look like

html > body {font-family: Garamond, ‘Times New Roman’, Times, serif;}

A generic font is always listed last to stop the browser defaulting to the browser default font.