What probate actually is
Probate is the legal process of proving a will and getting the authority to sort out a deceased person's affairs — paying their debts, closing their accounts, selling or transferring property, and sharing out what's left. The official document that gives you that authority is called a grant of representation.
Banks, building societies, the Land Registry and pension providers often won't release money or transfer assets in a sole name until they've seen this grant. It's their proof that you're the right person to deal with the estate.
A grant of probate and letters of administration do the same job — they're both grants of representation. Which one you get just depends on whether there's a valid will naming someone who can act.