PageRank is a neat algorithm that recursively weights incoming links based on their own incoming links. So a link from HN has more weight than from a small blog. And how does it know that HN should have more weight? Well, because there are many important sites pointing back to HN. And how does it know that those sites are important? And so on...

But AFAIK Google no longer uses PageRank, or at least not the same version as we know it.