Entailment or free tail is the process in which a property cannot sold, devised by will, or otherwise done anything with by the owner. The property passes by law to the heir of the owner upon his death. Entailment was used to keep properties in the main  line of succession. the heir of an entailed property could not sell the land, or give it to say an illegitimate child.
In this context Mr Cunningham has been telling Atticus and the children about his property is entailed.