Memory cells record information for your immune system about how to fight and destroy viruses and the like that plague the body. When a disease strikes the body the memory cells instruct the body on how to produce antibodies. Once created, these antibodies are released into the bloodstream. Then the antibodies find the disease and destroy it. If a disease that was not previously encountered is introduced into the body, then the immune system (hopefully) destroys it and the memory cells record how it was done.