A web page's PageRank is a number from 1 to 10 (as shown in the Google Toolbar) that is calculated by Google for that page. This PR value is basically what Google deems to be the importance of your page when compared to other web pages.

Google uses a complex PageRank algorithm to calculate each page's PR based solely on the quantity and quality of inbound links to the page.

PageRank isn't linear, it's logarithmic, making it a lot harder to boost a page's PR from 5 to 6 than from 4 to 5. The PR shown in the Google toolbar isn't the actual PR of the page, just the integer value.

If your site is in a competitive field, a 5 might be an excellent PR. If it is listed in a non-competitive field you need to aim for a higher PR. In general though, when it comes to PR higher is always better.

PageRank plays a role in how highly a page is ranked in the Google search results for a given search term. Everything else being equal, the page with the highest PR will be ranked highest.

Of course just getting your site into Google is only half the battle. It does no good to get listed unless your site shows up near the top of the search results. That is where working on your site's Page Rank (PR) comes into play.