Cumulative extremism is when a person begins to have very strong beliefs about something, and these beliefs become stronger and stronger over time. Imagine you really like a certain type of toy, like a dinosaur, and you start to talk about it a lot. Then you see other kids who also like dinosaurs, and you start to talk to them more and more about them. Soon, you start to think that everyone should like dinosaurs as much as you do, and you become very upset when they don't.
This kind of thinking can happen with bigger topics too, like politics or religion. If a person starts to believe very strongly in one political view, they might start to seek out other people who think the same way. They might read books, watch videos, and go to rallies that reinforce their belief. Over time, their beliefs about the political view become stronger and stronger, and they might start to think that everyone who doesn't agree with them is wrong or even evil.
Cumulative extremism can be dangerous because it can lead people to do extreme things, like violence or terrorism, in the name of their beliefs. It's important to remember that not everyone will have the same beliefs as you, and it's okay to respectfully disagree with someone else's opinions.