As per the Wikipedia article on the subject, SEO is the process of improving the visibility of a website in search engine results, specifically the natural/unpaid/organic results.
What is the result of SEO?
- Your site is more easily found
- Pages within your site are more clearly described to searchers
- Bottom line: increased sales due to increased traffic.
Is it better to do SEO before or after a web site is published
The answer can only be “before”. There are types of SEO that can be done after the fact, but it all depends on how well your data is organized.