Search Engine Optimisation (SEO) is the practice of increasing the quantity and quality of traffic to your website through organic search engine results.
People use search engines like Google, Bing, Yahoo, etc. to browse the internet every day. In the UK, over 80% of search is done through Google.
When building a website it is therefore extremely important that it is built with organic search in mind from the outset.
Adding this to your website after the development is complete is never quite as effective or as efficient.