On-page SEO covers things like site content, image optimization, internal linking, headers, title tags, and metadata, to name a few. The majority of on-page variables are associated with elements that are visible to website visitors.
Technical SEO covers things such as site speed, structured data, canonicalization, Hreflang, and XML Sitemaps, among many others. It is all about how search engines index your site and how efficiently they crawl your data.