SEO stands for “search engine optimization”. It means preparing a website so that search engines like Google can understand it well and show it higher in their results — which brings more visitors who are already looking for what you offer.
Imagine a huge library where the librarian decides which book to hand people first when they ask a question. SEO is making sure your “book” is clearly titled, well organised and easy to find, so the librarian recommends it near the top instead of burying it on a back shelf.
A big part of it is technical: the site should open fast (which is helped by Caching), have a clear structure, a proper Domain and tidy behind-the-scenes labels that tell search engines what each page is about. Good technical SEO is usually built into a website from the start rather than added later.