It allows you to search through a large number (nearly 3 million) of master level games. It allows you to study openings. It will tell you things like what moves are most common after 1.e4 e5 2.Nf3 Nc6, or any board position. This can help you as you research openings, either before, during, or after your current games. You can get more information about it here:
http://support.chess.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=687
http://support.chess.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=686
What is this and how does it work?