The Alternative Fonts field allows you to set a font for the entire page.
Fonts are an interesting part of web development. You may have a font installed on your local computer that gives you all the styles and typography for your designs but someone else might not have that font.
When they view your page online the font may be missing and when the browser can't find that font it will chose a substitute font.
There are many ways to handle this situation but a relatively safe way is to specify alternative fonts.