By getting your immunizations you have protected not just yourself, but also the vulnerable members of the population. If 84% of your community gets vaccinated against a specific contagious disease, then the odds of protecting the unvaccinated or immunocompromised, especially the elderly and young children, are favorable. This is herd immunity.
Why is it called herd immunity? Think of it as safety in numbers, or a hedge of protection. Surround yourself with armed mercenaries or a wall of people, and it will be pretty hard for an enemy to attack you.
Here’s a visual demonstration on how it works: