SEO stands for "Search Engine Optimization".
Every business who has a service or product to sell online wants to be as highly ranked in a search engine as possible. The higher ranked a website is, the more traffic and business they will get. However, most consumers who enter a search into Google (or Yahoo, or Bing) often only open links to websites that are on the first page, or maybe even page 2, of any search engine.
There is fierce competition between companies and business that are selling similar products to be ranked number 1, and there are many different ways to become so. For instance, if the website is a trusted source i.e. government websites; The age of the website i.e. a website that has been around for 10 years is going to rank higher than one that is only 2 years old; the speed of the webiste and how fast it loads; and relevance to what was searched for, which brings us to SEO.
All search engines will list their primary results based on the relevance to the user, so where a site ranks in a search is essential for directing traffic to a site. The best way to acheive this is by using keywords and phrases throughout your website. This can be done by creating an FAQ page (frequently asked questions), Product/Theme specific pages, creating Twitter, Facebook and possibly Instagram links and maintaining regular communication with consumers via your website. One of the best ways to promote and increase rankings on search engines is to write a regular Blog of all things, even remotely, related to your service or product. This way you can put as many keywords, phrases and questions as you like and linking them via your own Twitter and Facebook pages as often as possible.