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.
Three different screen sizes and devices with the same website that uses a responsive designt.
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.
Using Media Queries For Responsive Design In 2018 contains a bunch of great examples for how to use media queries to achieve a responsive design.
Making Tables Responsive With Minimal CSS steps through a couple of ways to responsively handle tables.
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.
Fundamentals of responsive images shows the code behind several ways to make your web images resize according to browser window size.
ResponsiveViewer is a Chrome plugin in that allows you to view multiple sizes and devices in the same browser window. This is a handy tool to avoid constantly flipping between tabs or resizing the same tab to see the changes you are making to a design.