Ask any successful shop owner, what is the 1 factor that determines a shop success.
You will hear something like "Location, location, location".
The internet is not that different from brick & mortar businesses, location is also the most important factor for success online, but this time it's about your website location among the web search results, commonly know as "ranking". Like in brick & mortar, prime locations are very expensive and good locations cost quite a bit, the analogy online is "keywords", you can rank your website for prime keywords like "wedding ring" or good keywords like "wedding ring London".
Online getting good ranking for a keyword can be done in 2 ways: The first, is by directly buying a search ad to display your website above other results, this option is very expensive, as each click on your website can cost from $0.5 to more than $50. The second way is to simply rank your website for the specific keywords by hiring SEO professionals, SEO offers and solutions cost only a small fraction from what buying search ads cost.
Understanding the website traffic and it's origin, help make the website rank better in search results.
Like with any profession, with time and experience, we learn some tricks, thus we have a list of do's and dont's to help us get good results.
The advent of the social web had it's influence on SEO, thus now search engines also factors many social signals into their ranking algorithm.