HTML (Hypertext Markup Language) is the standard markup language used to create web pages and applications. The HTML source code is the format readable to humans and it is the only stage where we can modify the code before the compiler translates it into what you see in your browser. You can view your source code in any browser (right click + Inspect in Chrome and right click + View Page Source in Firefox). If you wish to modify it, it is recommended to use a plain text or code editor. The bulk of technical SEO is done in the HTML source code.