The average percentage per sale is the average commission that you make. In this case, for every sale that they make, you get half, and it’s the same story with the average percentage of re-bill. No matter what; when you sell, you get 50%. Typically, the average percentage for a sale and the average percentage for a re-bill are exactly the same. The Gravity, again, is basically how many different affiliates are making sales, in this case over 300, which is very good.
Affiliate marketing is also called "performance marketing", in reference to how sales employees are typically being compensated. Such employees are typically paid a commission for each sale they close, and sometimes are paid performance incentives for exceeding objectives. Affiliates are not employed by the advertiser whose products or services they promote, but the compensation models applied to affiliate marketing are very similar to the ones used for people in the advertisers' internal sales department.