From the Introduction:
The Mystara 2300 BC campaign setting is a dark and foreboding world, full of despotic oppressive rulers trying to hold on to their increasingly tenuous rule with increasingly brutal methods. It is a post cataclysmic world, where the golden age of yesteryear and the advancements of Blackmoor can still be remembered, first hand in some cases. It is a world brimming with suffering and struggle, but it is also a world of hope.
This mini source book is intended to be read by DMs and Players alike, in order to provide both with a high level overview of the setting. Cultural and racial secrets are not revealed here, however DMs may wish to limit the sections that individual players have access to.