Here is a better explaination of what it means:
http://en.wikipedia.org/wiki/Unfair_preference

Your trustee is saying that you showed preference to one creditior in favor of another.
Do you have an attorney representing you in your bankruptcy? If so, he/she should have told you that it would not be recommended to pay any "one" creditor over the other and there is usually a dollar amount (say 600.00). You also need to ask you attorney what you can and can't do to fix it. Be sure you have proof that you paid the rent and basic necessitites needed. There is a chance that the ex husband will have to give the money back. If your in good shape here, you will have to pay the money back.