An allusion is a reference to history, literature, or popular culture.
For example, there is a use of historical reference in chapter 23.
Mr. Ewell was a veteran of an obscure war. (ch 23)
It is possible that this is the Spanish-American war, because it would not have been considered as important as the Civil War or World War I. The Spanish-American war was fought in 1898. Mr. Ewell might have fought in it when he was young.
The fact that Ewell is a veteran is intended to partly explain his belligerent behavior. This might make us sympathetic, but not too sympathetic because it is not an important war.
Now, what other references to history, literature, or popular culture can you find in this chapter?