Archiving prospects is used when a prospect (a lead) is no longer being pursued. Prospects are free to have, but they do utilize space. Meaning, that the more you have, the longer it takes for them to load. Another reason would be that if you are sending lots of emails to your prospects that are not interested in receiving your emails, they are more likely to mark your emails as spam or they could be permanently deferred which will decrease your rating. When a prospect is no longer being pursued it is a good idea to archive them freeing up space for the prospects that are still active. 

To archive a prospect, hover over the Prospects tab and then click on Active Prospects

Each student is in their own box with their information and on the right side of their box click Archive.

When you click this, a pop up box will ask you if you are sure. Click OK.

After you click ok, the prospect will then be moved to the Archived Prospect list.

To view your archived prospect hover over the Prospects tab again and this time click Archived Prospects. Notice that your Archived prospects are in gray boxes.