What is a hung jury?
(1) A hung jury is one where the jurors have not come to a unanimous or clear decision. In the Tom Robinson jury's case, the vote would have to be unanimous. At least one juror voted for acquittal (at least on the initial vote).
(2) Specifically, the Maycomb jury does not allow women or African-Americans.
(3) At first, Scout believes Alexandra and Atticus are discussing the possibility of sending her away. But, after listening further, she discovers that they are talking about Calpurnia.
(4) Atticus says he is not too afraid, but "too old." He means this at least half-jokingly.