Tribal Member Gaming Payments are periodic (usually quarterly) payments of gaming profits, distributed to every registered member of a tribe that derives income from gaming operations.
As the Indian Gaming Regulatory Act, 25 U.S.C. 2701 (1988), requires that the interests of minors and legal incompetents must be protected in these distributions, it is quite possible that the trust you refer to was established to hold your share of these payments when you were too young to receive them yourself. I would suggest that you make inquiry of the Tribal government or the Trustee of the trust to learn more about the details of the trust.