The residence nil-rate band is an additional inheritance tax allowance, on top of the main nil-rate band, that applies when a main residence is left to direct descendants (children, grandchildren, and certain others). It is also frozen, and it tapers away for larger estates above a set threshold.
Combined with the main nil-rate band, and transferable between spouses and civil partners, the residence nil-rate band can significantly increase the amount a couple can leave before inheritance tax becomes due.