SEO is Search Engine Optimisation and is effectively optimising your website to make it attractive to the search engines. If a search engine thinks your websites is relevant and popular, it will rank it highly but there are also some strategic elements that need to be addressed in order to really compete on a high level. There are many aspects to SEO and these can be split into on page and off page. Ideally, unless you are very confident with these techniques, you should employ a marketing agency to assist you with your SEO requirements. A good SEO company can be hard to find and a lot of it comes down to trust, as often SEO work that has been carried out is not all that obvious to the website owner. The best way to find a great SEO company is to do your research and go off recommendations and proven results.