The custom menu widget is used to create sidebar menus on most sites in the Meerkat theme. On this site, you can see an example to the right called “Topics.” The widget itself has very few options- it mostly serves as a way to map a specific menu to a location in your sidebar, but can also be used to make a callout menu box. The links that appear in the menu are controlled via the menu editor, not the widget itself.
You will find your site’s Menu Editor by visiting
Dashboard -> Appearance -> Menus