Search Engine Optimization helps people find your content when they are searching for related terms.
Customer queries
When someone searches for "bike stores in seattle" or "best mountain bike" you can imagine the companies and products that would like to get their attention. SEO, and more broadly, search engine marketing, is a way to enhance your web content to get it in front of people looking for it.
The goal of the game is to understand what people want and give it to them. People are searching for information, and based on the keywords and phrases they use, millions of results are provided in a list starting with the most relevant according to the search engine.
Search results rankings
The first result for a query gets the lion's share of the traffic. Most people will click on it and likely find what they're looking for. Some will come back to the list and click on the next one and maybe the third. It's worth it to your business to climb the ranks and get the traffic.
Almost all will find what they're looking for on the first page, or try another query.
How it works
Search engines need to serve good results in order for people to use them. They use a varieties of factors to determine the relevance and quality of content, and the order to display results.
In the early days of search, keyword relevance had priority so pages that repeated a word the most would rank the highest. Since this became quickly gamed, engines had to come up with a better strategy to find out what people are really looking for.
Search engines crawl the web, clicking links to spider out to all visible sites and pages. They create a keyword index, but also keep track of which pages link to others. Pages that have the most links will rank higher, this is one of the bigger factors. Engines use use keywords from linking pages to help determine relevance of a page, so a highlight recommended page gets a boost from sites that talk about it.
How to do it
First you need to measure your current situation. How much traffic are you getting? What pages are people visiting? Are they buying your product or leaving afterwards? At what %?
There are tools to measure your traffic analytics and SEO tools to map out the keywords you're going to target and how well your content is ranking vs the competition.
Over time, keep an eye on the reports while making adjustments to the content. Create content that you think will attract the audience that is interested in your product. Think long term, someone who becomes familiar with your site now may come back to buy something later.
There are some things to do on your site, like have good structure and content. From there, you need to have a strategy to get links back to the content by reaching out to an audience that can help you spread the word.