Responsive design is a method of using media queries in Cascading Style Sheets (CSS) to ensure a site is usable on various devices with different screen sizes, from very small to very large.

Full Stack Python uses responsive design to improve readability across a broad range of devices that people use to read this site.
The Responsive Design podcast and accompanying email newsletter interview web designers who've dealt with creating responsive designs from both a blank slate and retrofitting into an existing site.
This site is entirely on responsive design and has many articles on how to achieve readability across devices.
Smashing Magazine's article from 2011 on responsive design - what it is and how to use it remains a definitive source for understanding why sites should be responsive.
Searching for a complete, step-by-step deployment walkthrough? Learn more about The Full Stack Python Guide to Deployments book.