Users can enter a review when viewing an item on the site.  When they leave a review for an item, either the "Item Rating Notification Receiver" or the "Default Order Receiver" will receive the notification email.  The email gives you the item review information and a link to the "Item Reviews" admin.  From there, you can approve, un-approve and even delete item reviews  Approving a review will allow it to display for other users when they are viewing the item.