The remainderman is a person who will inherit an estate at some point in the future, much like a beneficiary. The main difference is that property can be inherited by a remainderman while a person is still alive through the transfer of interests within a trust.
Another way of using the term is when there is a beneficiary who does not want to inherit something, so they pass it along to another person. The person whom the beneficiary passed the inheritance on to would be called the remainderman.
If a trust is established over property, which dictates what should happen to the property upon the demise of the current property holder, the person who is next in line to inherit would be called the remainderman. In this way, a trust can ensure that no beneficiary along the way sells the overall interests in the property so that the property will be left intact and handed to the next inheritor.