Before we have a vaccine, there may be a disease that spreads readily. Most children got mild, moderate versions of the disease. Occasionally some would get severe reactions. With complications of these diseases, some children would die. Most of these children were previously healthy.
Once the vaccine was given to a large population, one thing was noticed: the likelihood of children getting these diseases dropped significantly in communities. The children were not likely to get the disease going forward. And if by chance they did, it would be a mild form of it. They are now “immune”. Vaccines give immunity without making children suffer the high price of natural infection.