This is a portfolio page. This is where you can list items from a particular WordPress category, and then folks can filter them by the sub-category. For example, you could display items from a Photo category that has the subcategories: Cityscapes, Landscapes, Galleries and People (as shown below).
As another example, if you are a designer or web developer, you could create a Portfolio category and then create sub-categories, such as Websites, Logos, Code, Brochures, Print, ect. You can be a creative as you like in creating your sub-categories.