In 1955, Rosa Parks, an African American woman from Montgomery, Alabama, refused to give up her seat on a bus to a white person, an act of defiance which violated the segregation laws of Alabama. Rosa Parks was arrested and her arrest led to the Montgomery Bus Boycott and, a little over a year later, a Supreme Court ruling which forced Alabama to desegregate its buses.
Rosa Parks is a hero firstly because her act of defiance was a moral stand, against the injustices and violence of institutionalized racism, and secondly because she took this moral stand at a time when the Ku Klux Klan were operating in Alabama (often with impunity given their close links with the police), bombing the homes of black social activists.
Rosa Parks's decision not to give up her seat for a white passenger is also often credited with providing the spark that gave life to the civil rights movement. Indeed, the Montgomery Bus Boycott was organized by a young pastor by the name of Martin Luther King, Jr., who of course rose to prominence as the figurehead of the civil rights movement.
In summary then, Rosa Parks was heroic because she stood up (or, rather, sat down) for what was morally right, because she did so at great personal risk, and because she helped bring about greater, lasting change.