PageRank is an algorithm developed by Larry Page and Sergey Brin that ranks web pages in search engine results. It works by evaluating the quantity and quality of links to a page, determining its importance in the vast web ecosystem. The core idea behind PageRank is that more important pages are likely to receive more links from other websites. This algorithm was a key innovation for Google, helping it deliver more relevant search results and revolutionizing the way information is accessed online.