This is required for cases where the beneficiary has an Immigration Court case that predates the marriage. It is a simple statement stating that the marriage was entered into for bona fide reasons and not illegimate purposes. Consult an attorney to help you craft the statement.
There is much more to your wife's case than the bona fide marriage exemption. If her appeal to the BIA is pending, you are going to want to move the Board to remand the case back to the Immigration Court so that she can apply for adjustment of status, otherwise you run the risk of the Board dismissing her appeal and ordering her removed while your I-130 is pending. I suggest you consult with an experienced immigration attorney to help you navigate what can be very treacherous waters.