Search engine optimization (SEO) is the practice of increasing the quantity and quality of traffic to your website through organic search engine results.
Search Engine's mission is to serve up relevant and quality results to its users. It can't understand websites in the same way that humans can, so webpages need to be designed so that Search Engine can understand what they're about - otherwise your chances of getting found in the search results are slim to non-existent.
Considering the fact that 90% of users searching the internet do not look any further than the first page of the search engine, and that 70% clicks on the first three results only, it is obvious that the position on the first page of the search engine is the only goal to strive for when optimizing a website.