Dystopia, or a Dystopian society is society where there's great injustice and suffering.
Usually this term is used in order to describe a totalitarian or a post-apocalyptic society in one word.
In a dystopia there's usually a single ruler which is very cruel, brutal, egoistic, and has complete control over everything.
The people live in poverty and suffering, often being tortured and made to do things that they don't want to.
This type of society can easily be said that is one of the worst possible scenarios that can happen to a society, or the whole humanity.