Technical SEO is about the configurations of website and server. It deals with website crawling, the site maps, server redirects etc.
Part of technical SEO is to work on the robot.txt file where the webmaster can mention the speed with with they want the search engine to crawl their website for indexing.
It also includes developing XML and HTML site maps for the website and uploading them in case of a new website. The site maps give a view of how content on website is organized.
Taking care of accurate page redirects if there is a server issue or a page is moved is also very important and part of technical SEO.