Responsive Web Design is a mixture of a handful of technologies and techniques, ultimately used to produce websites that respond to changing devices and rendering environments.
Some of the core tenets of responsive web design are fluid widths, flexbox layout, and responsive images, and guess what... you'll learn all of them in this mini course. My aim in this mini course has been to expose you to those parts of HTML and CSS that are tied to responsive web design (and that's why I didn't cover them in the individual courses for HTML and CSS).