The Curtis Act of 1898 was a law made by the grown-ups who ran the United States at the time. They wanted to take land that belonged to Native American peoples and give it to other people who weren't Native American.
This law made it so that the government could take land away from the Indian tribes that lived in what is now Oklahoma. They said they were doing it because they wanted the Native Americans to start living like other Americans and learning their ways. But really, they just wanted the land for themselves.
The law also made it so that Native American children had to go to schools that were run by the government, not by their families or tribes. This way, the government could teach them their ways and make them more like other Americans.
The Curtis Act of 1898 was not fair to the Native American people. It took away their land and their ways of life, all in the name of making them more American. But the Native American people had been living on this land for a long time before the grown-ups came and took it away.