The foundation search engines actually read
When Google visits your site, it doesn't see it the way you do. It reads the underlying structure — the code, the links, the labels — and tries to understand what each page is about and how it all fits together. SEO-ready architecture is the practice of building that structure so it's easy for a search engine to read.
You can have brilliant content and still rank poorly if the architecture is a mess, because the search engine can't make sense of it. It's like a library with no catalog and no shelf order: the books are all there, but nobody can find anything. Architecture is the catalog and the shelves.