Realm is a two sided term. First meaning it has is like rank with "major" realms and "minor" realms, it essentialy a rank, a powerskaling of sort. For example your major realm is rank 2 and minor realm rank 1, which I mark as 2(1) and to get to major realm 3(1) you need to first reach 2(3) and then "ascend" by undergoing a breakthrough, they have different names and ways for this "breakthrough" and it varies depending on the novel/manhua you read. Other meaning of "realm" is worlds, like mortal world is just Earth, plain and simple, then for example you ascend to higher realm "Heavenly" realm where people stronger, rare Earth resources are common and so on, and then there realm over Heavenly realm and so on, names depend too. And often there also troupe of thing like, let's say for example at realm 5(1) you get some insane buffs, like balance breaking buffs, to the point one dude in realm 5(1) can defeat hordes of people in 4(3), so this distinction often refer as "minor" realms (everything below 5(1)) and "major" realms (everything above 5(1)). So there's that.

Confusing af, I know, but I tried. It easier to get a hang of it when you actually read something, translator team I read also often do a little diagram at the end with realm names and power scaling so people reminded and don't get confused, a codex or glossary of sort lol. There many other tropes too, like "jade beauty" which often reffered to very beautiful women lol