Page rank is basically a measure of how many sites link to you.
In the Google directory (which is DMOZ data) they order the pages according to page rank, while Dmoz does it alphabetically.
You can get a basic overview here:
There are other factors that determine where on the SERPS (search engine results pages) your site will appear but page rank is one influencing factor.
Pagerank also can determine how much of your site is indexed.