Search Engine Optimisation (SEO) is the term used to describe the process a Web designer uses, in order to improve the likelihood of your Web site being 'ranked well' by Search Engines (i.e. appearing higher up the list of search results).
The objective is to increase the number of visitors to your website, which therefore might help to increase the number of 'leads' or 'conversions' (or whatever term you use to describe 'interest generated').
It's not for the benefit of people who already know about your website — they know where to look — it's for the people who don't know about it.
SEO is no guarantee of overnight success — it may take some time for your site to rank well in a list of search results.
Depending on the subject, a website may not feature at the top of the results list, or even within the first couple of pages of results — it depends what words and phrases people use when they search for something, and how many websites relate to the subject, among other things.
A good website design will be well structured and have well-written content. It will also aim to match key words and phrases used within the site to those which potential visitors are likely to use when searching. Plus, it will be designed so that it can be correctly 'indexed' by the major Search Engines.
At this point it's worth considering that Search Engines can (and do) change the methods by which they 'rank' pages in the list of results. This means any 'tricks' used to improve the ranking of a website today may not be of any benefit in the future, and some tricks can even get your website removed from the search listings. So, a well-designed website stands the best chance of being ranked higher.
When we go through the process of deciding what content your website needs, we'll also consider the wording used, and any 'keywords' and 'key phrases' which might be appropriate, so you might think about some of these.
Most importantly, remember — the content is meant to be relevant to visitors, not just created for Search Engines to find. After all, it's no good being at the top of the search results if the content doesn't make sense.