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.