Users will need Rate permissions in order to rate assets.
Users can Rate their assets on a One-to-Five Star rating basis.
Select your asset or multiple assets, and then go to the File Info Panel on the right. At the top of the Panel you will be able to assign the selected Assets with the rating of your choosing.
If multiple Users have rated the Asset(s), you will have the option to 'See all ratings' and the Average rating will be reflected in the Panel as well as if you hover your mouse over the asset thumbnail.