During your SEO lifecycle for any website, you always try to achieve organic rankings for your websites. Different search engines follow different algorithms and consider different factors to rank a website.
To rank organically in Google search engines, you should know that Google determines the relevancy of a user's keyword or query over 200 factors. And out of which Google green bar know as PageRank is also on of the major ranking factor for a website.
To show google page rank on a web page or to add actual Google PageRank on your websites, you can use FREE Page Rank Button from https://www.pagerankbar.com
PageRank is calculated on the bases of how many links/websites are linked to a respective page or website. All incoming links adds to the PageRank of the linked page/site.
All google tools such as Google Adwords also uses PageRank as one factor while showing paid links in Sponsered Links section in Google search results.