Once you have enabled Activity Ratings, you have three options that you can select within the Activity Ratings tab.
Allow all Users to view individual Ratings on Activities
Tick this box if you would like Users to be able to view individual Ratings on Activities.
Allow all Users to view average Ratings on Activities
Tick this box if you would like Users to be able to view average Ratings on Activities.
Users must leave a comment when they give a Rating which is equal to or less than...
Choose the number of stars for this option using the dropdown menu. 
Alternatively, you can choose the 'do not enforce' option to make the comment box a voluntary field when the User is rating the Activity.
If you make changes to any of the settings detailed above, remember to click on the Save button.