To be oppressed is to be exploited or systematically harmed by others. People who have no freedom are oppressed.
Being oppressed is a negative and unfair condition. The United States was founded because we felt oppressed by the rule of England. African-Americans, Native Americans, women, gays, and other minority groups have all been oppressed by prejudice and unfair laws. So people who are oppressed tend to be angry — and for good reason. Oppressed people tend to fight for their freedom, and they often get it.