PageRank is Google’s ranking software that calculates the relevance of a webpage against a search keywords entered. Google analyses both the number of incoming links and more importantly the quality of the referring webpage to generate a 'ranking' between 0 (low-relevance) and 10 (high-relevance). (The ‘quality’ of the referring webpage is an measure of how authoritative it is on the subject matter).

In a nutshell: the more high-quality-webpages that link to your webpage, the higher its PageRank.