SEO (search engine optimization) is really just following the rules of how search engines read webpage content which ultimately decides which sites are most relevant to your search and serves them up to your request. 

If done correctly, SEO should position your website on the front page of search engines for the optimized keywords within a few months. Cost can run on the low end of $279 a month to thousands of dollars a month depending on the size of the site or really the amount of keywords you want to rank for. 

You have probably heard somewhere about SEO or at least been pitched an offer on the best SEO tactics available for  hundreds if not thousands of dollars a month. So what is SEO? They are selling you a service. 

There are a lot of factors at play, one being  the level of competition. In small town USA there may be 5 plumbers in the town and if your site is search engine optimized you will start showing up where they are immediately.