Followers are the number of readers following your blog and blog posts comments.

Your followers receive an update, either on their Read Blogs page or via email (or both, depending on their settings). There are several different ways that readers can start following your blog. http://en.support.wordpress.com/followers/

Go to your site stats page and click “Totals, Followers & Shares”
Click the Blog link under the number and the Site Stats » My Followers page will open. Click the Email Followers link.
http://wordpress.com/#!/my-stats/
http://wordpress.com/#!/my-stats/?blog_subscribers
http://wordpress.com/#!/my-stats/?blog_subscribers&type=email

The subscriber stats on your site stats page show you all the users that have active email subscriptions to your blog using the built-in email notification system on WordPress.com. They don’t include users that subscribed using a third-party subscription service or using RSS.

For clarity likes and shares numbers are misleading when you are talking about page view stats. Your followers and anyone with a WordPress.com blog who is logged into WordPress.com can "like" and "share" your posts in several locations such as the Reader without ever clicking into your blog and creating a page view stat. Those who have username accounts only but no blog can become followers and click the "like" and/or "share" buttons. Those who do not have accounts cannot use the like button. http://onecoolsitebloggingtips.com/2012/07/09/wordpress-followers-likes-and-stats/