For each user you refer to Host-A, you will earn 1000 MB of bandwidth. To refer users, they must sign up for Host-A through your referral link, which can be found when you log in to your account. Put this link on your website, forum signature, etc and watch the free bandwidth come rolling in.
Even if you don't promote your referral link, you might earn some bandwidth anyways. If a user views your profile page, or downloads a file from your account, and signs up afterwards, you are credited for that. Who knew rewards came this easy?