SEO stands for Search Engine Optimisation and is effectively optimising your site in such a way to improve its rankings on sites such as Google, Yahoo and Bing. Every website should be built with SEO in mind but often, especially older sites are lacking some of the basic elements let alone the advanced SEO techniques. If your website seems to be lacking in traffic and you cannot find your site on the first page or so on Google for your mains search terms, you need to start looking at your SEO.
If when you had your site built, you did ensure it was SEO friendly, that may have been great to start with, but over time this will need to be tweaked and new pages optimised to allow your SEO to build.
Getting SEO wrong can be worse than not doing it at all as you can find your site penalised and therefore removed from the search engine listings entirely. Simple mistakes such as putting text in white on a white background or having duplicate pages with the same content on can get your site blacklisted. When doing SEO be sure to use a company or individual that has the relevant experience to carry out the work.