Greenskeeper theme comes with Custom widget created specially for the theme. To set the Widget go to Appearance > Widgets.

List of some included Widgets

1. Custom Categories
2. Custom Menu
3. Layout Builder
4. Meta
5. and many more…

To use widgets, drag your widget into the sidebars and footers

Kindly refer to How to Setup and Use Widget.

Additional Information : WordPress Widgets – Codex for more information on how to use WordPress Widgets.