Unrest indicates a city's tendency to produce Rebels, citizens who generate no File:Icon Food.pngFood, File:Icon Production.png Production, or File:Icon Gold.png Taxes (like all citizens, they consume File:Icon Food.png, and Rebels still generate File:Icon Mana.png for races that do so. See Citizens for more details). Unless you maintain a tax rate of 0 and have only cities of friendly races, eventually unrest will be a problem your empire will need to deal with.