What is PR?

I see it as a stair, or better, as a ladder, where if you are PR1 you are on the first step (or the last if you prefer) and if you are PR 10 you are on the last (or first, depending where you look from).
That is, if your blog or website is very important, people have to pay to be with you, while if you are on the bottom you have to pay the people to be with you.
A small detail that makes a huge difference...