Responsive design means creating web pages that adapt to different devices.
Before smartphones and tablets became popular, web designers created fixed width pages that worked on the most common screen sizes – usually 1024 pixels wide by 768 pixels high.
This all changed around 2013 with the massive increase in iPhone use to browse the web.
Source: What is responsive design? : Iteracy
Most web pages follow a common set of sizes and layouts, which you’ll recognise even if you aren’t aware of them. For example, did you know that nine out of the
Source: Web page size and layout : Iteracy