On Dec 1, 1955, in Montgomery, Alabama, Rosa Parks, an African-American, refused to give up her seat on the bus to a white passenger, as local law required. She was arrested. A few days later the black community in Montgomery began a bus boycott.
What's a boycott?
A boycott is when a large group of people refuse to take part in, or make use of, something as a way of showing their disapproval.
Because so many black people rode the bus, a boycott would cause the bus system to lose a lot of money.