In the beginning search engines looked at your website, looked at the meta keywords and a few other on site factors and trusted that that was what your site was all about. The problem with that was that not all site owners are as honest as you or us, so they used these factors to manipulate sites to the top of the search engines which may not have deserved to be there as they were often not the best answer to the users query.
Then along came Google, a search engine created in two young graduate's garage. Google was a search engine with a difference. Google used inbound links as the main factor in its ranking algorithm. The theory was that it is more difficult for site owners to manipulate other people's websites than their own. Google considers each link to a website as a vote of confidence. But it doesn't end there, as all links are not created equal; the more authoritive & more relevant the linking site the higher the value of the vote.
As we all know Google has been a huge success, delivering more relevant results to users and keeping user experience as the number one priority. Google's algorithm has developed and changed over the years and become ever more complex but the basics are still the same:
You need a continued influx of relevant sites linking deep within your site to optimised landing pages.