Feynman Diagrams on Srednicki's book.

[S_klogW]

I am reading Srednicki's Quantum Field Theory recently. I have thought over the descriptions of the Feynman Diagrams on his book in chapter 9, but I still can not understand what these diagrams mean and how do they help us to calculate the right-hand side of the equation 9.11 on the book. Is there anyone who can help me?
See http://web.physics.ucsb.edu/~mark/ms-qft-DRAFT.pdf, page 73. And the equation 9.11 is on page 72.
Thanks for your help.

 

[sgd37]

read on till feynman rules if anything isn't clear come back