What does Scout mean by entailment in To Kill A Mockingbird?

Entailment is an old-fashioned form of bequeathing real property. Entailed land (aka Fee Tail) can only be inherited by the owner's issue (legitimate children). It was a way to keep an estate intact for multiple generations. Since the land could not be sold or easily borrowed against it left some individuals rich in land but still heavily in debt. Only four US States recognize Fee Tails and most European nations have done away with them. The few nations that still recognizes entailed estates only recognizes existing ones and do not allow new ones to be created.

Cunningham had been to Atticus to resolve an entailment problem. I suspect that the land Cunningham had was entailed, meaning that he was not the clear owner of it and thus could neither sell it nor mortgage it to raise money.

Another possibility is Cunningham risked losing land if he was not a clear heir to the original owner (i.e. oldest child or was illegitimate).

Later on when the lynching party gathers outside the jail, Scout mentions that entailments are bad.