Original: The agent stimulates the body's immune system to recognize the agent as foreign, destroy it, and "remember" it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.
Simplified: The agent in the vaccine stimulates the body's immune system to recognize the agent as foreign, destroy it, and "remember" it. Then the immune system can more easily recognize and destroy any of the microorganisms that it later encounters.