A childhood illness is an illness that you usually get as a child. You can catch it very easily. Once such a disease has appeared, many people get it at the same time.
If you have had the childhood disease once, you usually can't get it a second time and are immune to it. That is why childhood diseases are so rare in adults. A childhood illness usually passes without major problems, but sometimes it is very dangerous.
Sometimes we talk about childhood diseases in new machines or products. This refers to temporary problems that occur at the beginning.