Staged reforming is when you try to make big changes to something, but you do it slowly over time instead of all at once. It's kind of like building a sandcastle - you start with a small pile of sand and then slowly add more as you go, instead of trying to make a big castle all at once.
When people talk about staged reforming, they usually mean trying to change something in a way that won't upset too many people or cause too many problems. For example, if you wanted to change the rules at school so that everyone had to wear uniforms, you might do it in stages - first, you might only make the students in one class wear uniforms for a week, to see how it goes. Then, if everything went well, you might expand it to more classes, and so on.
The idea is that by making changes slowly and carefully, you're more likely to get everyone on board with the new idea, and less likely to cause any big problems. It takes patience and persistence, but in the end, it can lead to big changes happening in a more harmonious way.