SEO is an abbreviation of Search Engine Optimization. This is the process of structuring a web page so that it is found, read, and indexed by search engines in the most effective manner possible.
This makes your web site and its content attractive, relevant and visible to search engines and web searchers.
Why is this important? Consider what it would be like if no one could easily find your place of business, or even your telephone number. Most businesses could not continue for long in such a situation.
The same thing can happen with your web site if people cannot easily locate it. Traffic volume, if it existed at all, slows to a crawl. Potentially valuable customers never even know you are there.
Let us consider first a web site that I know of which is extremely difficult to find. This particular site is for a fine restaurant in the community where I live.
I didn't know the address of their site, so I went to Google.com, entered the name of the restaurant into the text box, and clicked the search button.
Many links were listed, but none seemed to have anything to do with the restaurant. To the search terms I added the name of the city where the restaurant is located.