Google measured relevancy by determining how many websites were linking to a particular website or article. The website with the most links back to it was deemed the authority on a topic and placed at the top of the search results. This was a huge step up from manually cataloged websites and is what set Google up to become what it is today.
Google gained a huge audience in the early 2000's and eventually became the de facto standard for internet search. Initially, many analysts questioned how Google was going to monetize their offering. Its website was incredibly clean, had no banner advertisements and its rankings were strictly based on relevance. As Google grew, it leveraged its search capabilities to offer dozens of products and over time it created and advertising model called Google Ad Sense.