Pagerank is a simple calculation that is a combined indicator of the quantity and quality of links in other pages pointing at your page.
They´re using it as a basis of all their other algorithms. For example, they can see a pagerank value of your page for a specific keyword.
The toolbar of Google shows a green bar that is a logarithmical indication of the total amount of pagerank that they calculated for your page.
Since pagerank is not the only algorithm, but merely the most basic algorithm that they have, you can't say that the highest pagerank page will rank highest. As in the example I just gave, for a specific keyword pagerank is different. Thus toolbar may show a high
PR page rank very low.
It's not something to worry about, (though it is a fun subject) but it does make it easier to do a quick review of a website.