What does it mean when websites have a PR number?

The PR number stands for Page Rank. (Google)
It's part of a new SEO Craze where you can buy websites already owned by someone and already are performing well on google.
The lower the PR#, the better the Page ranks.

Thanks

That's not entirely true.
It's a higher Page Rank that will result in better ranking in Google's search results.
But there is a difference between what is called Toolbar PageRank, which is what we users can see if we have Googles toolbar installed, and the "real" pagerank that Google uses internally to calculate the relevance of the websites for a concrete query.
Many SEO experts regard the Toolbar PageRank as nothing more than very weak indicator of the possible PageRank of a site. The fact is that the Toolbar PageRank is updated very infrequently, only every 3 months or so, so a lot could have happened, both good and bad to a websites ranking since the last update of the Toolbar PageRank, which anway is only an approximation of the real one.