So let's imagine you have just launched a new website for your business. How do you think people will find it?
You can tell them through marketing, advertising, etc and with a bit of luck they'll visit it (if they remember the domain name).
What you really want to do is use the power of internet search engines (i.e. Google/Yahoo/Bing) to bring visitors to your website for you. In order to do this there are a number of things you have to do such as telling the search engines your site exists and telling them what your site is about - this is search engine optimisation.
So when someone searches for 'website design bournemouth' the search engine offers them your website to visit.