"What to do you mean by that" means what?? :-?
If there are two sites A and B and if B links to A then Google
interprets that link from page B to page A as a vote by page B, to page A.
Google analyses how important page B is and accordingly it gives
importance to page A.
Thus google considers all links that page A receives from various webpages
and accordingly ranks ( give importance to page A). PR varies from 0 to 9
and higher PR pages have higher importance.
So if two webpages have PR 3 and 6 respectively then every other sites will
be more inclined to link to site of PR 6 than site of PR 3.