A foundational piece of the puzzle when it comes to Search Engine Optimisation is how easy you make your website for search engines to access, digest, and categorise the content on your web pages. Search engines visit your web pages with bots whose job it is to find your content and add it to their index in a categorised way.
Technical on-page SEO deals with how you make it as easy as possible for those bots to know exactly what your content contains, what it's about, and who it's for. If your content is more readily accessible to the bots, and presents information in a way that allows the bot to categorise your pages easily, then your pages are more likely to be technically possible to rank in a results page.
Great on-page SEO won't make your site rank well on it's own, but without it every page on your site will struggle.