Answer:
In the first diagram the weakened pathogen (weaker version of the disease) is inserted into the bloodstream.
In the second diagram the antibodies kill the weakened pathogen. When the immune systems encounters a new disease it "remembers it".
This is why in the third diagram, when the "real" pathogen enters the bloodstream the immune system can effectively kill the disease. As the immune system "remembers" how to defeat the weakened version it can more effectively kill the "real" version.
