The statement on the part of Atticus that "Jem's definitions are very nearly accurate sometimes" is meant to set a playful tone because earlier Jem had said that entailment meant getting your tail stuck in a crack.
Explanation:
The word entailment is significant in this part of the story because it means that Mr. Cunningham could not just sell his property to get cash and pay his debts. He might have a farm, but he does not have to right to sell it a large part of it and dispose of the money as he wishes. Therefore Walter and his father are poor, and kind of "stuck in a crack" when it comes to affording to take a lunch to school like Miss Caroline expects.
