The under-the-hood SEO work: site speed, crawlability, structured data, architecture.
Technical SEO is the stuff your users don't see: can Google crawl your site efficiently? Do pages load fast on mobile? Are URLs clean? Is the sitemap up to date? Are you handling duplicate content correctly? It's the plumbing. When technical is solid, everything else works better. When technical is broken, content and backlinks can't compensate.
Most sites have significant technical issues that silently cost them rankings. A technical audit every 6-12 months pays for itself. Site speed especially: slow sites don't rank well, full stop.