SEO is an abbreviation for a concentrated term known in the industry as Search Engine Optimization. Basically SEO is used to help search engines find your website easier using keywords, terms, headers, relevant page content and a good clean and crisp site design which easily allows search engines to read the content on your web pages.
Google the biggest search engine in the world reveals little information on how their search engine works and so even to professional SEO specialists the task of optimizing a website for good rankings in the search results is still very difficult. Especially when the search engines have a tendancy to update the way their search engine works every few months.
A person who is good at SEO and has optimized their website well will be seen higher up the search engine results pages, for example on the top of page 1, when one of their keywords is typed into the search engine. Bad SEO will result in lower page rankings on the search engine, will not be found easily by people searching the internet and in some cases search engines will ban the website from being viewed from their results page entirely.
So basically get a professional to do it, never attempt SEO yourself.