When a person dies, somebody has to deal with their estate by collecting in all the money, paying any debts owed at the time of death and distributing the remaining estate to those people who are entitled to it.
Not just anybody can distribute the estate.  An application needs to be made to a Probate Registry to provide one or more people with the authority to do so.
The Probate Registry issues the document, which is called a Grant Of Representation.

1. Probate
Issued to one or more of the executors named in the deceased's will.

2. Letters of administration (with will)
Issued when there is a will, but there is no executor named, or when the executors are unable to apply, or do not wish to be involved in dealing with the estate

3. Letters of administration
Issued when the deceased had not made a will, or any will made is not valid.

A grant of representation is necessary because most organisations (banks, building societies etc.) will not release the money due to an estate without the necessary authority.