page label
A page label is the number a reader shows for a page, as opposed to where that page physically sits in the file.
In more detail
Open a book and the numbering does not start on the first sheet. The foreword runs in small roman numerals, the body starts again at one, and an appendix may run A-1, A-2. The sheets themselves, meanwhile, have simply been counted from the first.
A file works the same way. The position of a page is fixed by where it sits; the label is what somebody reads, and the two are set apart on purpose.
What it does on a real document
A document states where the numbering restarts and in what style, and a reader shows that in the box where the page number goes. Typing 12 into that box then lands on the page marked 12, not on the twelfth sheet.
The same label can be drawn into the page itself, so what is printed at the foot of the sheet and what the reader shows are one and the same.
Where it comes from, and why
The habit is older than any file format: a printed book has numbered its front matter apart from its body for centuries, because the front matter is written last and its length is not known while the body is being set.
Reading software took the habit over so that a page cited in a footnote could be reached by typing the number the footnote gives.