A grant of probate is a legal document that the executor of a will can use as proof of their right to manage the legal and financial affairs of a person who has died.
Banks and other financial organisations often ask to see a sealed copy of a grant of probate before they’ll let an executor access any accounts. This is especially true for accounts with lots of funds, although the threshold is different for every organisation.
When someone dies without a will, the person sorting out their estate will need to apply for letters of administration instead.