sponsor,bank letter是什么

Your sponsor or bank letter is documented proof that your DCF is
correct. This form must show that you have at least $29,180 of support for
one year (2 semesters).
请问这个sponsor or bank letter 是什么呢？国际卡的卡号还是学校申请费的支票号码是不是呢？请多多帮忙！非常感谢！