Menu Builder is the final section within the Engagement App tab and allows you to choose what items get shown in your app menu (and what order they are shown in). This is the menu you will see down the left hand side when you log in to your app. E.g. below:
Build the menu by clicking on the icons of the features you would like to include in your menu (see image below) and once they have been added you can re-arrange the order by simply clicking and dragging the widgets to re-order.
Hide what you don't want to include by closing the eye icon, and you can rename the widget by simply clicking on the pencil icon (then typing the new "name" before saving).