NRMB stands for Non Runner Money Back. NRMB means that if for some reason your selection does not run in the selected race on which you have placed your bet, your stake would be returned to your account.
Non runner money back applies to previous ante post races where the final declarations have been decided and where your stake will be returned if your selection doesn't run. William Hill also offer NRMB as a concession in major races prior to the final declarations.