Modernism in Roman Catholicism refers to a movement that happened nearly a hundred years ago where some people started to question the traditional teachings of the Catholic Church. It was like they were saying, "Hey, maybe some of these old ways of thinking aren't quite right anymore!"
For example, some people in the modernist movement thought that science and new discoveries might show that some things in the Bible weren't literally true. They also thought that it was important for people to think for themselves and not just blindly follow what the Church was telling them to do.
However, the Catholic Church didn't like this new way of thinking and some leaders in the Church even said that modernism was a bad thing. They believed that the traditional teachings of the Church were important and should not be changed.
So, to sum it up, modernism in Roman Catholicism was a movement where some people wanted to question old teachings and think for themselves, but the Church didn't like this and wanted to stick to tradition.