A font is the set of characters that makes up a certain type style. All of the text on a web page is set in a specific font, such as Times New Roman or Arial, with a defined size and color.
When you choose a design for your site, the text on your pages will use the font (or set of fonts) selected by the creator of that site design. You can change characteristics of the text (such as its color) or change the font family (from Times New Roman to Arial, for example) with your editing toolbar.