The big brother movement was a group of people who thought that it was important for older boys and young men to help younger boys who didn't have fathers or who were having a hard time. It was like having a big brother, who could help you with things or just be there for you.
People who wanted to be part of the big brother movement would volunteer to be a big brother, and they would be matched with a younger boy who needed one. The big brother would spend time with the younger boy, do fun things together, and help him with any problems he was having. The idea was that the big brother would be a good role model and friend, and that the little brother would learn from him and feel supported.
The big brother movement started a long time ago, in the early 1900s, and it's still around today. Lots of people think it's an important thing to do, to help young boys who don't have a lot of support in their lives.