typographic point
A typographic point is the unit printing measures with, worth a little over a third of a millimetre.
In more detail
Seventy-two of them make an inch, which puts one at about 0.35 millimetres. Everything on a printed page is counted in them: the width of the paper, the depth of a margin, the height of the lettering. A sheet of 472 by 692 of them is a sheet of about 167 by 244 millimetres.
The unit is small enough that a whole page comes out in round whole numbers, which is what a trade working by hand wanted. A sheet of A4 is 595 by 842 of them. Lettering set at 10 is the size a letter is usually written in.
What it does on a real document
Every measurement inside a PDF file is written in these, counted from the bottom left corner of the page. Nothing in the file is stated in centimetres, and nothing is stated as a share of the screen.
That is what makes a document come out the same size wherever it is opened. A page does not grow because the screen showing it is larger, and the printing works receives the very figures the person who laid the page out chose.
Where it comes from, and why
Printers measured in points long before any of this. A piece of metal type was cast to a point size, and a page was built by adding up pieces. The exact size drifted from one country to the next until computer typesetting settled it at one seventy-second of an inch, which is what a PDF file means by it.
See it on a finished document
A sheet that measures what it says A sheet prepared for the printing works