Search Engine Optimization (SEO) is a range of activities to improve your visibility in the search engine rankings/listings.
Historically you would create a site and wait for visitors. This is no longer enough. Would you leave your brochures in the cupboard?
There are millions of websites. You need to be visible.
There is little point in creating a website that visitors cannot find.
The website has become one of the most important marketing channels. If your competitors are found before you, then you LOSE! Natural (Organic) Search Rankings are very effective at driving traffic and generating quality leads.