Five years ago, the world-wide-web was a much, much smaller place and any website around then was a "big fish in a small sea". Nowadays, with hundreds of websites being added to the WWW every day, competition is intense. Search engines are far more choosy about who they select to be on the first page of search engine results. Today's search engines such as Google, Yahoo! and bing analyse the content, architecture and relevancy of that content at every level on every page and in microscopic detail.
Each search-engine places slightly different emphasis on aspects of your website to enable them to rank your website and so place your website on the top, in the middle or at the bottom of the their pack (SERPS). They use things called "robots" to do that and data collection process is completely automated.
In search engine optimisation (SEO) you need to have the proper website design, content and technical attributes in place to rank well and be top of their list for that search term or phrase. The search engine robots (who report back to their respective search-engine owners who sent them) will look for these criteria and if they are missing, inappropriate or ineffective, then it will not add to your overall score of page importance or "page-rank".
We have described SEO or Search Engine optimisation in it's simplest form. There is far more to this specialist area of website development but it gives you an idea of what needs to be done to a website to be at or near the top of a search-engine results page. Note: Search Engine Optimisation is the same as Search Engine Optimization. The latter being english-american (en-us) whilst the former is en-gb.