Fill it out and send it back ASAP. It may not mean a thing ...but it may mean everything down the road. You should be keeping track of all correspondence from now on.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .