What is technical SEO?
Everything that lets Google crawl, render and index a site without friction: speed, mobile layout, clean URLs, sitemaps, redirects and structured data. Invisible when it is right; a ceiling on everything else when it is wrong.
Technical SEO is the part nobody notices when it works. It decides whether Google can fetch your pages, render them, understand them and store them, before the quality of the writing even comes into it.
In practice that means a site that loads quickly, lays out properly on a phone, uses clean and stable addresses, tells Google where everything is with a sitemap, forwards old pages with 301 redirects, and describes itself with structured data. It also means not accidentally blocking the pages you want found, which happens more often than anyone admits after a redesign.
The reason it matters is that it sets a ceiling. You can write the best page in Ireland on a subject, and if Google cannot render it, or it takes an age to load on a phone, it will not rank the way it deserves to.
Technical SEO does not win rankings on its own. It removes the things that stop the rest of the work from winning them, which is why it is usually the first thing we look at on a site that should be doing better than it is.
Search that compounds, run by people who show their work.
Strategy, technical, content, links and local, planned together and reported against the numbers your business cares about.