The nil-rate band is the amount an estate can be worth before any inheritance tax is due. It has been frozen at £325,000 for a number of tax years. Any unused nil-rate band can usually be transferred to a surviving spouse or civil partner, potentially doubling the threshold available on the second death.
Inheritance tax is generally charged at 40% on the value of the estate above the available nil-rate band (and residence nil-rate band, where it applies).