A link to a page is regarded as an indicator of popularity and importance, with the value of this link increasing with the popularity of the page linking to it. The key idea is that a web page is important if other important pages link to it.
Thus, PageRank is a popularity contest: it assigns a score to each page according to the number of links to that page and the score of each page linking to it. So it is recursive: the PageRank score depends on PageRank scores of other pages, so it must be calculated by an iterative process, cycling repeatedly through all the pages. At the beginning, all pages are given equal scores. After a few cycles, the scores converge rapidly to fixed values, which are the final PageRank values.