What Is Herd Immunity?

It's when you have a group of people, say 50% of the population of a city, and they get vaccinated for a particular disease, it indirectly produces protection to unprotected individuals (those that haven't been vaccinated). Take a disease that spreads in a chain among people, for example. If a significant portion of the population are immune to it, the chain breaks and it's much harder and less likely for the disease to spread as wide as it would normally do.