No, it means browsing through each individual note in the notebook. Skimming the content of each note, which is much easier when you have 12 notes in a subnotebook than when you have hundreds in a more general notebook.
I'm anticipating someone will next ask why I don't have hundreds of notebooks in stacks to break it all down. I used to, but a lot of the stacks were too similar and it made things confusing. I research hundreds of topics a year, and I find a nested directory structure much more efficient for my brain to find things it only vaguely recalls.
It seems clear from this thread that EN could now add the nested notebooks due to the recent changes in the base code, but they still don't understand why it's important to some of us, and that's why I've replied here. Seeing that EN doesn't get it, but Nimbus Note, Notejoy, Notion and OneNote do, I accept that it's time to cancel my subscription and move to one of the others.