Search Engine Optimisation (or SEO) is the process of ensuring that search engines find and position your website higher than the thousands of other websites containing the relevant search term. In other words, SEO improves the visibility of your website.
When you search on a particular term, the search engine tries to rank the pages it finds that match your search term by relevance and importance. Many people will only look at the first page of results returned and some will not scroll down past the first few items on the first page. Therefore the higher a search engine ranking, the more likely your website is to be seen. This is the aim of search engine optimisation.
Some of the factors that affect the relevance and importance of a page are: