A more thorough explanation:

A residuary beneficiary is a person who receives any property from a will or trust that is not specifically left to another designated beneficiary. This means that if the testator (the person who made the will) did not name a specific person to receive a certain asset or property, that asset or property will go to the residuary beneficiary.

For example, if a person's will states that their house goes to their daughter, their car goes to their son, and the rest of their property goes to their friend, then the friend is the residuary beneficiary. They will receive any property that was not specifically left to the daughter or son.

However, it is important to note that a residuary beneficiary can also be involved in legal disputes. In the case of In re Estate of Burger, a Pennsylvania Supreme Court case, the Court found that one of the residuary beneficiaries to a will rendered it partially invalid because the residuary beneficiary exercised undue influence to obtain property from the testator. This means that the residuary beneficiary used their power to manipulate the testator into giving them more property than they deserved.