Google is known as a search engine, but that is not where they derive most of their revenue. Their business relies on selling ads. Google figured out how to auction ads on the Internet and became one of the most valuable companies in the world based on their ability to broker the sales of ads. They used the metric of impressions or the number of people who viewed the page with the ad to price ads. Each impression costs a small amount, but they quickly add up. By auctioning these small chunks it enabled them to sell the same ad to many people.