The multiverse is divided into three sections, for want of a better term. There are Earths which are alternate timelines where things may have turned out very differently, but where the laws or nature work pretty much like they do on our Earth.
There are Otherworlds, where magic is prevalent and legends are real (if not exactly the way you know them). Finally there are Netherworlds, or Hell-dimensions. Each of these three world-types have their own god-like beings who strive more-or-less consciously to further a transcendent agenda. There are rare cases of worlds which do not fit into the cosmology.
You move between worlds through gates. They can generally not be made, but must be discovered or taken from those who have claimed ownership of them. Some powerful entities can make gates, but those are rare.
There are some transplanar organizations vying for dominance.
The Vitane: Sort of the like the United Federation of Planets from Star Trek. Well-intentioned explorers trying to guide the cosmos without being imperialist bastards.
The Divine Empire. Feudalist and expansionist. They’re ruled by an aristocracy which claim to be related to gods by blood.
The Nighthost: Deserted Hell-warriors who have created a sprawling barbarian society which Conan might find himself at home in.
Each of these have adventuring-party-esque groups, but the game doesn't presume that you necessarily belong to any of them.
There's a lot more stuff, but those are the broad strokes. Let me know if you want to know more.
The book comes with a good deal of new backgrounds and professions. About half of them are setting-specific (you're working for one of the organizations mentioned above in some capacity), while the rest are more generically suitable for a supernatural mystery game (Monster Hunter, Occult Detective, Escaped Test Subject etc.). Then there are ancestries, which would be what D&D calls 'races'. It is an agreed-upon fact that all creatures with a soul are human. There is just more diversity in a whole multiverse.
The ancestries are:
Arvu: They're basically elves, but elves who lean towards the 'children of magic' side of the typical 'elf package'. They gain physical traits which show their dharma or roots.
Dreygur: At first I thought that these were the orcs, and in a sense they are. They're humans who have been shaped by hell-worlds (and hell-masters) and are startlingly scarred because of it. They're kinda like orcs if they decided to revolt against Sauron, and are usually more freedom-loving than evil.
Huldra: The dwarf-analog, kinda. They're not short vikings, but like dwarves they're industrious, creative and deeply aligned with earth and minerals.
Jana: Doesn't have a clear fantasy rpg analog that I can see. They possess great social skills, cultural adaptability and empathy. They're kind of 'larger-than-life' humans.
You're not required to pick an ancestry, nor is it given that you have to go all the way. It is strongly implied that you can have 'arvu blood' while stilling being what us earthlings would recognize as human. It's a matter of degrees.