Ok kiddo, have you ever played with toys that you didn't want to use anymore? Sometimes you need to put those toys away so you can play with your new toys.
Now imagine that instead of toys, we're talking about guns and weapons that grown-ups use. Disarmament is when those grown-ups agree to stop using their weapons and put them away.
After they put their guns away, they need to find something else to do. Just like when you put your toys away, you might need to find a new activity to do. Demobilization is when the grown-ups stop being soldiers and go back to being regular people.
Once they're regular people again, they need to figure out how to live without being soldiers. They might need help starting a new job or finding a place to live. Reintegration is when they become a part of the community again and start to live a normal life like you and me.
So basically, disarmament, demobilization, and reintegration are all about grown-ups putting their guns away, stopping fighting, and becoming regular people again with help and support.