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.
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.