Something seditious works against a government. When countries overthrow authoritarian dictators, often the people engage in seditious protests until the dictator has to leave.
Seditious, pronounced "si-DI-shes," comes from the Latin seditionem meaning "civil disorder, dissention." A seditious act rebels against a government or authority. In some countries just saying seditious remarks against a government can land you in jail, especially if what you say is likely to get people to rise up against the government. If you plot against the current power structure in any way, you're being seditious.