It is relevant to all other web-pages on the WWW indexed by Google.
Seen by itself it rates the specific page you are on in comparison to other pages on the WWW and indexed by Google.
The digit in the Toolbar goes from 0 to 10. Its the toolbar pagerank.
You have 50% or a pagerank of 5. That is reasonably good.
Google just decided to rank webpages based on the importance and the number of links toward each page. The more links the better, and the higher the pagerank of the page of that link the better.
They use a formula for counting and evaluating these links. This adds up to a specific number for each webpage in the Google index. This number is the actual pagerank. Google then tiered the actual pagerank numbers of all the indexed webpages into a logarithmic scale.
For example you may need 6 to 10 times the same amount and quality of links to go from toolbar pagerank 5 to 6.
For the few websearchers who use the toolbar and look at it and understand it, it could be a way of rating your site.
For example a PR0 page may be frowned apon.
A greyed-out toolbar means the site is not in the Google index.
As you read through the different threads on this subject here, you will understand that it helps your own pagerank to get a link from a high pageranked page.
If that link is motivated by a link-text description, you tend to end up higher in rankings for the words used in that link-text and more so the higher the pagerank.