Technical SEO is often poorly defined and can often refer to several disciplines, therefore it can be helpful to break it down into “onsite technical SEO” and “offsite technical SEO”.
Onsite technical SEO
The first purpose of onsite technical SEO is permitting basic access to your content (not blocking it conclusively) so that search crawlers can add your pages to their index. The second purpose, which is often overlooked, is to make it easier and more rewarding for search crawelers to access. This can be done in a number of ways such as having a smaller file size, greater content to code ratio, more efficiently organised inter-page relationships, etc.
Offsite technical SEO
Offsite technical SEO is concerned with link detox, which is the process of disavowing (removing) toxic links pointing towards your site. These links can range from unnatural links, for instance purchased links, to backlinks received from spam/virus sites.