Brother Jack wants the narrator to avoid understanding what?

The narrator thinks over the training which had been more work than classwork in school as he had daily reading, discussions, and speakers to hear at night. Jack asks him about Hambro. He tells the narrator that he has heard good reports and warns him to not let the material master him. That is not to get too pressured on understanding the theory.

The narrator thinks over the training which had been more work than classwork in school as he had daily reading, discussions, and speakers to hear at night. Jack asks him about Hambro. He tells the narrator that he has heard good reports and warns him to not let the material master him. That is not to get too pressured on understanding the theory.