The feedback page contains all feedback or grades you've ever given or received. You can view them by going to your profile and clicking on My feedback.
There are two tabs here, one for feedback you've received.
And one for feedback you've left.
This is a great way to keep track of what others are saying about your work. This will help you improve your skills by finding out what are the most common errors or mistakes you've made that are being frequently brought up in your feedback.
It's important to review this from time to time to see if you need to work on some areas of your writing, etc.