Here is a Short Description of what a Vaccine is, and what it does.
The Immune System acts like an army - It waits for an invader (Pathogen) to attack.
Some Pathogens are so Deadly we have to have a jab. The jab contains a dead version of the pathogen, which is harmless.
When this is put inside the body, the immune system thinks it is damaging the body (although it's not) because its antigens are still intact, so it attacks it.
Once it has attacked it and killed it, the dead virus no longer exists in the body. The immune system remembers the dead Virus though, And stores info about the it, and what it took to kill it, so it is ready to respond if the real virus ever enters the body again.
Herd Immunity is when a large amount of people in the same community get a pathogen because it is spreading for person to person. When parents avoid having their children vaccinated, this more often.