CSS Media Queries allow web developers to create responsive websites that can adapt to different screen sizes and device types. They allow developers to write CSS rules that will only be applied when certain conditions are met, such as when the viewport width is a certain size or when the user is using a
From Wikipedia
Media queries is a feature of CSS 3 allowing content rendering to adapt to different conditions such as screen resolution. It became a W3C recommended standard in June 2012, and is a cornerstone technology of responsive web design (RWD).