What is the meaning of Retainer?

Under the Bankruptcy and Insolvency Act it is common for the Trustee in Bankruptcy to ask for a' Retainer' before he accepts an appointment. There is also a special type of retainer called a Third Party Retainer, whereby funds are put up that secure the fees and disbursements of the Trustee but are returned to the party putting up the funds if the Trustee realizes enough out of the estate to cover the fees and costs and, in the case of a proposal, returned to the party putting up the funds if the proposal is refused by the creditors or not approved by the Court.