Typography is where you will control and customize the bulk of all your different types of Text from Normal Text to Button Text.
Each type of text comes with a full range of customizable features such as Font, Size, and some even offer the ability to add shadow to the text.
You'll notice when you make changes to a section, the changes will be immediately displayed in the theme preview within the editor. For example, if you change the font type for the 'Normal Text', you can see where the changes will be applied in the preview on the right.
Note: You can find Typography in a few other areas in Theme such as in Header for the Navigation Bar.