Imagine you are playing a game with your friends, and one of your friends wants to make a rule without asking everyone else what they think. You might feel upset because you didn't have a say in the rule, even though you're playing the same game!
The phrase "nothing about us without us" is kind of like that. It means that when grown-ups are making rules or decisions that will affect kids or groups of people, they need to ask those people what they think and what they want.
For example, a school might want to change its dress code. Instead of just making the change without asking the students, they could ask the students what they think and what kind of dress code they would like. This way, everyone gets to have a say in what happens, and everyone feels heard and respected.
So, "nothing about us without us" means that it's important to include everyone in important decisions, especially the people who will be affected by those decisions.