Heirs at law are the relatives entitled to inherit a person's property under state law when that person dies without a valid will, in an order the statute sets, typically starting with a spouse and children. In Tennessee, real property vests in the heirs at law immediately at the moment of death, rather than waiting on probate to assign it.
Why it matters
If a house passed to more than one heir at law, every one of them generally has to sign to sell it.
In Tennessee: Tenn. Code Ann. § 31-2-103. This can work differently in another state.
General information, not legal or tax advice.