Site architecture is the structure of your website. It's important for SEO for two reasons.
Firstly, search engine 'crawlers' can only find and index pages that are linked to from another page. That means that every new web page or blog post you create should be part of an interlinked structure.
Secondly, the structure of your website tells the search engines how your content is organized. If your content is organized into topic clusters, for example, search engines can see that your website has 'topical authority' for that topic.
The best site structure for SEO purposes is three-tier structure, where tier one is the home page, tier two consists of 'category' pages, and tier three consists of individual topic or product pages:
Here are some useful articles on site architecture: