A class action suit is one initiated by a group of customers who have a beef with HP because of a problem with some or all of HP's products. They join together as a group to have the clout to get results.
If they are asking you to join the suit, Id' recommend doing it. It costs nothing and you might get a few bucks out of it, or some free stuff.
If you do agree to become part of the class action, you give up any right to sue HP on your own.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan