SEO continues to be a cryptic topic for the average person.
This is partly because of the centralized and algorithmic dictation of its functionality, but that's a post for another time!
SEO stands for Search Engine Optimization, and simple refers to how easy to is for a search engine to index a web pages information and therefore how prevalent it will be when users search for related terms contained on a page.
In short it's is important for a website because no matter how great your content is, if your SEO website has poor SEO internet users are unlikely to come across your website when searching.