There are three different categories of optimization you can perform to improve your website in search:
Technical SEO is the optimization of technology involved in Google crawling and indexing your site. This includes domain architecture, technology stack, robots.txt, server codes, redirects, page speed, internal links, etc.
On-page SEO is the optimization of the content and HTML of your site’s pages themselves. This is a more granular view of the elements on the page, which help Google understand the topic and intent of the page. This includes the words on the page, the URL, title, header tags, images, meta tags, etc.
Off-page typically refers to links, although it can also include citations.
Technical and on-page SEO are crucial — you’ll never rank to your site’s full potential without clean, optimized on-page and technical SEO. Fortunately you should largely have control over these elements, which are the direct result of how you build your website and its platform.
Technical and on-page SEO both become much more complex as you scale a website (increasing the number and size of pages), and involve more technology.
Off-page is the element you least control in SEO. It requires convincing another website to provide positive signals to your site, which doesn’t happen without a good cause.
Links have perhaps the most ranking value of any single signal. They’re absolutely critical to ranking in competitive search.
Securing a link from an authoritative website in a relevant manner isn’t easy. Google’s original algorithm succeeded because it used links as a core ranking signal, which no other search engine used at the time.