A sub-affiliate is someone who joins a two-tier affiliate program after being referred to it by another affiliate. For example, if a person registers to betcart Affiliates based on your recommendation, he/she becomes a sub-affiliate.
Once your sub-affiliates start attracting new players, you will earn additional revenue from their referral activity. Therefore, as well as earning commissions on your own affiliate activity, you earn commissions on sub-affiliate sales. In this sense sub-affiliates are treated the same way as regular affiliates.