A realm is an area that is ruled by something. If you are a controlling cook who doesn’t like anyone to do anything else in the kitchen, then the kitchen is your realm.
Realm comes from the French for kingdom, but since there aren’t so many kings or queens left in the world, you’ll more often find it referring to non-royal spheres. That could be the sphere of a special kind of knowledge, like the realm of biology, or the political realm. Or it could be an area dominated by some group. Little Italy was famously the realm of the mafia; Hollywood is the realm of film stars.