PageRank (PR) is Google's method of calculating a web page's relevance and importance. This is done by taking into account a number of different factors, but most importantly it looks at the quality of sites linking to it.
PageRank is scored out of ten. There are almost no sites which score a full 10, even google.co.uk only scores a 9.
The fact that Clearbox IT's website has such a high PageRank is a testament to the success of the websites that they build.