A bond trustee is an institution which has the fiduciary responsibility of administering and enforcing the terms of the bond indenture. A bond indenture is the contract between the bond issuer and the bondholder.
A trustee has the resources to manage the distribution of the funds to the bondholders, to keep up with and distribute the required bookkeeping and statement information to the interested parties as well as regulators like the SEC. If there is a violation of the contract, the trustee must report it and act in the best interest of the wronged party.
That may mean sitting on the opposite side of the table from the issuing entity in legal proceedings and arbitration talks.