Remember that a Feynman diagram shows the way that particles interact:
This example shows two protons repelling each other. Remember that a photon of an electromagnetic wave is the particle that carries the force for electromagnetic forces.
We start to read this example in the bottom left hand corner. A single neutro appears. Then something happens that produces a proton and two other particles. The neutron must have decayed.The other two particles are created from a W-. This is the particle responsible for the weak nuclear force.
This example is called beta decay, because the electron produced is ejected from the nucleus.
This is another version of the same beta decay. This time it shows the quarks that make up the neutron at the start. It allows us to see that beta decay causes one of the quarks to change flavour.
