What is debenture trustee

Very simple, a nominee like parents or wife after the holders death.

Trustee stands for holding something in trust , it's mandatory if company issue secured debentures /deposit,so trustee holds or manage charged property in trust for benefit of security holder ,also requires for logistics convenience (one deed is executed in favour of trustee) so no need to execute many deeds in favour of investors which may change over a period of time + trustee takes care of interest of investors and timely payment)