SEO is an abbreviation of Search Engine Optimisation. 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.