New RWD : Media Queries aren't explained

In step 20 of Learn CSS Flexbox by Building a Photo Gallery, we are asked to create a media query. However, media queries aren’t explained neither in this challenge nor the ones prior to it. A brief explanation would be less blunt.

Thank you for helping make FCC better. Bugs can be reported as GitHub Issues. Whenever reporting a bug, please check first that there isn’t already an issue for it and provide as much detail as possible.