The 3 boxes below the featured area slider on theme's demo site are called Promo Boxes. These boxes allow you to draw special attention to any post, page, or general URL you'd like.
To set up your promo boxes, navigate to Appearance > Customize > Promo Boxes Settings.
Here, you can enable/disable the promo boxes and input your various promo box preferences.
For each of the 3 promo boxes, you have the option to enter a title & subtitle, select an image, and input the URL you'd like that specific promo box to link to.