What did radical Republicans do?

Answer 1

They are known to have played a part in Reconstruction and the passing of the three amendments-the 13th Amendment, the 14th Amendment, and the 15th Amendment. They had 'radical' ideas, therefore becoming known as the Radical Republicans.

Answer 2

I'm assuming that you mean the Radical wing of the Republican Party which rose to power in the years following the American Civil War?

Radical Republicans supported punishing the South for their role in the war, particularly through using military occupation by federal troops (known as Reconstruction). Radical Republicans generally supported giving freed slaves rights, and using military force to protect these rights; during Reconstruction, many African-Americans were elected to federal office. Moderate Republicans, by contrast, supported reintegrating the former Confederacy into the United States as smoothly as possible. The moderate Republicans ultimately succeeded in ending Reconstruction during the extremely close Presidential election of 1876, when they were able to trade the withdrawal of Federal troops in exchange for Rutherford Hayes' election in the House of Representatives, who were forced to decide a disputed result in the Electoral College.