Once you've located this widget, click on the down arrow indicated by the
green arrow to see the widget settings. The title is the title which will be displayed at the top of the widget in the sidebar it is located in. By default none of your post categories will be hidden. To hide certain categories, simply check the boxes located next to them. The purpose of this widget is to display a post tree. (You can find more information about widgets in general at
https://codex.wordpress.org/WordPress_Widgets)