Imagine a wall of soldiers lined up on a battlefield. If every soldier is properly armed, they will be able to hold their ground against an invading army. Now, imagine that a few soldiers slack off and leave their shields at home on battle day. It’s not hard to grasp that if enough soldiers are missing their shields, the army as a whole isn’t going to stand much of a chance defending itself.
This is exactly how vaccines work. Each individual person who gets vaccinated is a single shield against an invading horde of terrifying viruses lurking all over the world. If everyone is vaccinated, everyone’s shields will form one single impenetrable wall, and diseases won’t be able to spread.