If you name one or more trustees to manage your trust (co-trustees), the substitute co-trustee is the person you name to replace a co-trustee if they are unable to work with the other co-trustee(s) to manage the trust.
For example, let’s say you name Chris Hoffman and Georgina Thomas as co-trustees and you name Raul Henriquez as the substitute co-trustee. If Georgina cannot serve as co-trustee with Chris, then Raul will step in and Chris and Raul will be co-trustees.