What does it mean to say that a novel contains "levels of meaning"?

These levels could also be understood as themes.

For Example:

To Kill A Mocking Bird has many themes. (Levels of Meaning)

- "when these words are put together to convey a meaning it is known as a sentence. a sentence is a group of words that makes complete sense. placing of words in a sentence is very important. the second sentence is a question."