There is no greater influence over the success of a website than its placing in the rankings of search engines.
The visibility of a website is the ease with which potential customers can find it amongst its competitors; crucial for the success of a website and often key to the success of a business.
Search Engine Optimisation is the range of tasks and techniques which improve search engine ranking of the individual pages of your website when people use particular words in their searches. There are properties of a site as a whole which influence the search engine's view of whether your site is important and useful to the user. Attention to these things encourages the search engine to place your site higher in the search results.
Building a website without search engine optimisation is like making a TV advert and not buying any airtime.