Technical SEO
Technical SEO is the work of ensuring search engines can crawl, render, understand, and index a website correctly.
In full
Technical SEO covers everything that determines whether a search engine can access and interpret a site, as distinct from whether the content is any good. It includes crawlability, meaning bots can reach pages through links and are not blocked accidentally; indexability, meaning pages are eligible to be stored in the index; correct canonical tags so duplicate versions of a page do not compete with one another; an accurate XML sitemap; structured data; site architecture and internal linking; loading performance; mobile rendering; and secure delivery over HTTPS. These problems are invisible from the front end of a website, which is precisely why they persist for years on otherwise well-managed sites.
Why it matters
Content cannot rank if a search engine cannot reach, render, or understand it. Technical problems set a ceiling on everything else, and they are usually cheaper to fix than content is to produce.