Amazingly, this is one of the most important activities associated with the construction and marketing of your site. As you might expect, there is quite a "cottage industry" of SEO "experts" out there - with an incredible amount of bad (even damaging) advice and practice.
The first objectives of every site should be clear and informative content for the human visitors. However, if this content contains absolutely no mention of the very terms humans are likely to use in a search engine, you won't have many people reading that content. Search Engine Optimization is nothing more or less than adding or adjusting things on a site to make it more "searchable" - without damaging the content for the human users of the site.
As you would expect for any human endeavor, unprincipled scalawags have moved into this territory and attempted to break or bend the rules in order to get their client's sites listed prominently in the search engines. Equally, as you would expect, the search engines have all become aware of these unethical tactics and since they can almost never directly deal with the culprits, they penalize the sites in question by lower rankings. In some extreme cases, a site might even be "blackballed" and not appear anywhere within that engines listings!
DO make sure ALL of your search terms appear naturally somewhere in your site. DO have a bonafide SEO expert change the "behind the scenes" page coding to enhance your site's placement.
DON'T believe that an SEO can PERMANENTLY place your listing near the top. DON'T believe that META tag changes, keyword stuffing, text on same-colored backgrounds etc. will improve your site rankings - they might just get your site BANNED.
NetWorthy Designs performs expert, ethical and "accepted practice" Search Engine Optimization, and will provide this service as part of your web site development and marketing.