This video is the VERY basics of what HTML and CSS is, for the absolute beginner. HTML and CSS files are, quite literally, just text files. You don’t need any special software to create them, although a nice code editor is helpful. You can create these files on any computer and use your web browser to preview them during development. You can think of HTML as the content of your website: a bunch of text and references to images wrapped in tags. CSS is the design of your website. It targets the tags you wrote in your HTML and applies the style. Keeping these two things separate is key to quality web design.
Where should you go after this? Check out this series from Khan Academy.