Okay kiddo, so the Rio Branco Law is a law in Brazil that has to do with how the country's borders were decided. You know how when you play a game you have to decide where the boundaries are so everyone knows where they can play and where they can't? Well, it's kind of like that.
Brazil used to have some disputes with its neighbors, especially one called Venezuela, about where the border should be. So, in 1904, the Brazilian government made a law called the Rio Branco Law that basically said they were going to use a special method to decide the border once and for all.
This method was called the "utipossidetis juridis" and it said that the borders should be the same as they were when Brazil became independent in 1822. So the idea was that everyone had to agree on where the borders were at that time and that's where they would stay.
The Rio Branco Law was important because it helped Brazil avoid war and conflict with its neighbors over border disputes. It also helped to establish Brazil as a peaceful and stable country in South America.