Referrals are, simply spoken, users who you introduce by having them sign up through your unique and personal YTMonster registration link.
Your personal link can be found in the Referrals section found in the sidebar. You can then share this around the web, or with your friends and family.
If they are completely new to YTMonster, and they use your unique link when signing up, they will become a referral of yours. You can find a full overview of the benefits of getting referrals, and an overview of users you have already referred here.