Casual here just trying to understand what this setting is.

What the hell is a 'Realm'? I am getting the sense from the limited info I've read, that it's basically a big disk, sort of, or maybe a planet?
But also, do people just live normal medieval/feudal lives there, farming and such, or do they get food to magically teleported in from Sigmar etc? Like how does it work, how high fantasy are we talking here? Do all the races/species just mix etc?