On December 1, 1955, Rosa Parks boarded a bus in Montgomery, Alabama. Instead of going to the back of the bus, which was designated for African Americans, she sat in the front. When the bus started to fill up with white passengers, the bus driver asked Parks to move. She refused.
Rosa Parks invigorated the struggle for racial equality when she refused to give up her bus seat to a white man
so the joke is that he went back in time to make Rosa Parks go to the back to stop a racial equality move
in short, the joke is racism