What It Is
Before Google can show your website to anyone, it has to be able to reach the site, read what is on it, and decide it is worth keeping. Technical SEO covers everything that makes those three steps possible. None of it is about what your pages say. It is about whether the machinery underneath lets anybody read them.
How It Works
Google publishes a short list of technical requirements, and they are genuinely short: Googlebot, Google's crawler, has to be allowed in; the page has to load successfully; and the page has to contain content Google can index. Around those basics sits the rest of the craft. A secure connection (the padlock in the address bar). A site that works on a phone, since Google looks at the mobile version of a site first. A sitemap that lists your pages. A robots.txt file that is not accidentally shutting the door. Every page with its own title and description. Old addresses that redirect to new ones instead of dead ending. None of this is visible to a customer, and all of it is visible to Google.
The free way to check most of it is Google Search Console, which reports which of your pages Google has stored, which it has not, and why.
Why It Matters for Being Found
Technical problems do not make a website rank a little lower. They make pages disappear. Take a brand new site for a local clinic: modern, fast, every service described on its own page. Six weeks after launch, Google had never heard of the services page, the reviews page, or one of its articles. Nothing was broken. Nobody had handed Google the sitemap, so Google did not know where to look, and it kept rereading the handful of pages it already knew. The owner never sees a problem like this, because the owner already knows the address. This is why technical SEO comes first. Rankings, the map, and AI answers are all built on top of it, and a crack here quietly undoes the work above.