From the Super Mario Wiki
Frederick Douglass (born Frederick Augustus Washington Bailey) was an abolitionist who widely spoke out against the practice of slavery in the United States of America, earning considerable renown for his excellent oratory skills. He was born into slavery and was taught to read and write at a young age in secret. He eventually escaped after several failed attempts, and soon began a long career of fighting for the rights of black individuals, which earned him enough clout to meet with president Abraham Lincoln. In Mario's Time Machine, Mario happens to meet Frederick after travelling back in time to the White House in Washington, D.C. on January 1, 1863 (the day of Abraham's Emancipation Proclamation) to return Abraham's Spectacles.
Mario's Time Machine
According to Mario's Time Machine, Frederick Douglass was in the White House on January 1, 1863 (despite him actually visiting the president on August 10), where he was waiting to meet with president Abraham Lincoln. When Mario first meets him, he asks him if he is also interested in seeing the president. Frederick introduces himself and says that, while he has been waiting for a while, Mario is more likely to see the president before him. Mario ignorantly asks if it is because he is a plumber. Frederick interjects that it is because of their skin color. He says that he wants to see the president because he wants to support the president's abolitionist movement (despite him actually visiting the president over concerns about the treatment of black soldiers in the army). As Mario does not know what an abolitionist is, Frederick explains that it refers to someone who wants to end slavery. He also tells Mario that if he wants to be educated, he needs to have a Dictionary. Mario promptly obtains one, and Frederick tells him to look up the words "abolition", "prejudice", and "emancipation". Mario says that he has a lot to learn, and Frederick responds that everyone has something to learn, but also that it is dangerous for thinking people in the American Civil War. He also brings up that seven states seceded from the United States to preserve their right of self-determination. Mario asks why they should not have this right, and Frederick replies that the right of self-determination should not affect the lives of people; as someone who escaped from slavery, he is especially adamant that nobody should be able to buy or sell another human being. Mario finally understands his point, then asks him if the Spectacles in Mario's possession belong to him. He says that his eyesight is good enough, and thinks that they perhaps belong to Mary Todd Lincoln. He also remembers that she left a bolt of Cloth in his room, and he gives it to Mario while asking him to give it to her. Mario then goes off to deliver the Cloth.