The simplest meaning to understand SEO is that SEO helps you to expose your website well to the users of search engines like Google.
And how does SEO work?
By implementing SEO techniques such as:
- Creating unique, attractive and valuable content
- Correct and high-quality linking
- User-friendly design of a theme for your website
You can show your site on the Google results page higher than competitors and get more visits.
This was the simplest answer I could give you.