It has no official meaning. It generally means pages that expect to have the most traffic on a wiki, like the main page or the primary page for the topic, like the The Expanse page for the the 'The Expanse series of books/TV show.
High traffic pages tend to be targets for vandalism, because the vandal expects their vandalism to be seen by more people.
Aprogergely wrote: no, it seems to have been done regularly
There's different levels of protection.
If they've just stopped logged-out users editing, it might just be that the wiki has had a lot of vandalism in the past, and they're preempting any trouble. If they've set it to only staff that can edit, without it being a temporary measure, that's against the ToS (I believe). I think regularly protecting pages against signed-out users is normal, to be honest
It might be good to protect a "high traffic page" if it hasn't been created yet. There are pages that have been created a lot in the past, but have been deleted every time. Those pages could be considered "high traffic" even though they don't exist.
well, as far as I know, most of these never existed, but its true that it would not be out of the question for a potential vandal to create a page with that title, and that they would then be forced to delete it.
Another thing to note: Clever vandals will move/rename a page a bunch of times so you have to follow the redirect trail and delete all the redirects. So, to prevent that, it is usually a good idea to protect a page from move/rename (second section of Protect page UI) and editing.