I think he means that it is like the basic underlining theory behind the war plan, as brass isn't the most precious and valuable of metals so is a metaphor for something basic and fundamental. Well's that's my perception of it anyway
Here's a line I always found confusing:
When Fry is describing the P.E. Crew members around the table in The Cyber House Rules he gets to the end of the descriptions and Professor Farnsworth says "And Fry you've got that brain thing..." and Fry replies "I already did"..... Whaaa??? Is it supposed to mean that he is so stupid that he gets his grammer wrong or what?
Also nice idea for a topic Actor, I to love how the dialogue of the episodes has a much deeper comedic meaning that only the intellegent can understand or notice