Probate is the legal process of dealing with a deceased person's estate: their money, property and possessions. It usually involves applying for a legal document (a grant of probate, or grant of letters of administration if there is no will) that gives the executor or administrator the authority to collect in the estate's assets, pay any debts and tax, and distribute what is left to the beneficiaries.
Not every estate needs probate. Small estates, or estates held entirely in joint names that pass automatically to a surviving joint owner, can sometimes be dealt with without a formal grant.