With respect to African American history, John Brown represents the most extreme anti-slavery activist who was not himself a slave. John Brown also represents the idea that there were white people who took the side of the African Americans in their desire to be free (just as there were whites who supported civil rights later on).
John Brown was an extremely radical abolitionist. Some of the things he did were not necessarily all that radical. For example, he moved to a black community in the North because he wanted to help teach ex-slaves the things they needed to know. He also participated in the Underground Railroad. However, he is best known for two violent episodes. One came after he and his family moved to Kansas to participate in fighting against pro-slavery forces during the time known as “Bleeding Kansas.” While there, he participated in at least one massacre of pro-slavery people. The second episode is better-known. This was his raid on the armory at Harpers Ferry, Virginia in 1859. He intended to use the weapons to raise a slave rebellion. During this episode, he was captured and executed.
Thus, John Brown is significant for the fact that he was a white man who was utterly radical in his hatred for slavery.