OK, let me give you an example.
Say you have one website with PR 10.
PR 10 website links to a website with 0 PR. 0 PR site now will gain some stance on PR, becoming PR 3.
Now you have another website with PR 6
PR 6 Website links to a website with 0 PR. 0PR site will now gain some increment on PR, becoming PR 2.
Say you have One thousand (1000) PR 3 website
1000 PR 3 website link to a PR 0 website. That PR 0 website gain some increment on PR, becoming PR 1.
The basic idea is that when a website got a reference from another website, it's PR will be calculated on the basis of the PR of another site that give reference to it. The mechanism works exactly the same like thesis you do in master and postgraduates. The number of references and quality references come into play.