PageRank is a numeric value that represents how important a page is on the web. Google figures that when one page links to another page, it is effectively casting a vote for the other page. The more votes that are cast for a page, the more important the page must be. Also, the importance of the page that is casting the vote determines how important the vote itself is. Google calculates a page's importance from the votes cast for it. How important each vote is taken into account when a page's PageRank is calculated.
pr 3 is good. I believe pr is one of thousand of factors Google uses to rank a site in their search results.
I'm sure PR is only one of the many factors Google uses to rank sites, but seeing as how all the major sites have high PR I'm sure is a little more important than most other ranking factors.
Yes, pagerank is important because it serves as your website ranking on search engines. The higher your websites PR, that means you have been updating on your website. Having a high PR also helps you earn more traffic and therefore you can increase your sales.
For me, I think PR on your page means nothing - what's important is PR on other people's pages that will link to you. If your site have high PR, it'll help you do a link exchange easily with other people though.