In Chapter 6, Dill asked "Cross in it tonight?" to Jem and Scout. What does he mean by this in To Kill a Mockingbird?

When Dill says this to Jem and Scout, he is talking about
the moon.  Scout (as the narrator) has just mentioned that there was a very big full
moon out that night.  Dill asks the two siblings if there is a cross in the moon and
they tell him that only the lady is in the moon at that particular
time.

What they are talking about is sort of folk
traditions about what shapes you should be able to see in the patterns of light and dark
on the moon.  In modern US society, people sometimes talk about the man in the moon
because there is supposed to be a face there.  I have heard that the Japanese say you
should be able to see a frog there.  Apparently in Maycomb, you could see a lady and
sometimes a cross.