Well, the world is one set several centuries after the apocalypse. It's not necessarily the default Mutant Futures world, or Gamma World's setting .. it's an original as it were.
But, like those settings, humanity has managed to claw its way back up to medieval level technology. Society is on the level of the village, with limited trading and communication between villages. Most of the world is wasteland, inhabited by mutated monsters of all varieties. Only the bravest travel far, except in fairly large caravans (which are rare due to the sparsity of population). So it is a world of isolation, mystery, and danger.
The campaign focuses on a particular village that is fairly well off, and situated in the general vicinity of a large ruined city. Expeditions are occasionally made to the city, but few ever come back. The village is notable for being open and accepting of mutants so long as they are 'behaved.' Players could therefore be human, mutated human, mutated animal, or even mutated plant. As I noted, Mutant Future rules are used, basically as written, with a few exceptions (most notable is that the rules as printed allow androids, which I will not).
More detail than that is not available at start, as part of the premise of the campaign is exploration of an unknown world.