Search engine optimization (or SEO, as it is better known) refers to a number of different practices whose aim is to increase the visibility of websites both by restructuring on-site elements as well as through off-site techniques which help make it easier to find via search engines.
Why does this matter? It’s much like having a business without listing your telephone number or publicizing your address. It’s doubtful that any business could make it for very long if they were to do this.
The same goes for your website – if no one knows where to find you, it’s unlikely you’ll continue to get any traffic, if indeed you have much to begin with.