I think with everybody searching out good links many sites have increased the incoming links to themselves quite a bit. I know I doubled the number of links PR4+ since last google update. My PR stayed the same :(
I guess relatively the sites that did not actively pursue links since last update appear relatively much less important to Google. So If you had around 70 something PR4+ links before last update now you need about 140 for the same PR 5.
Imagine that you start from a random page somewhere on the web. Randomly choose one link on that page and follow it. Then randomly choose and follow a link on the next page, and so on.
The Page Rank of pageA is a measure of how likely it is that you will eventually find pageA on such a "random walk." So a PR of 10/10 on PageB means that every time you start a random walk, every place you start one, you will eventually find PageB.
At least, that's the pure theory they started with. They've been tweaking the math all over the place, but PR is still an attempt to measure how "connected" any page is to the web as a whole.
Guysmy, you might also think of it as a voting system that isn't "one site, one vote". Instead, sites that have themselves been "voted" as being important by their peers (where a vote is a link) have votes that count a lot more than a site that has few links itself.