The Follow button lets the people who care about your success--family, friends, former teachers--get updates from your university when they promote your achievements.
Located just under your profile image, you can encourage anyone to sign up with their name and email address and follow you on Merit. This way, they'll receive an email each time you earn an achievement.
It's a great way for them to stay up-to-date on the "good stuff" and spread the word; you can even suggest that future employers or your professors follow your achievements while you're at school.
You can see who's following you, find people whose achievements you'd like to follow, and block followers if needed by logging into your Merit account.