Media queries are a part of CSS that allow you to determine what styles should be applied to certain screen sizes. Rather than redevelop everything for a mobile view, media queries allow us to take what we already have, and alter it so it fits on a mobile screen.

To write a media query, you must start of with the @media declaration, then declare what screen size you want your media query to take effect on:

<style>
@media (min-width: 640px) {

}
</style>

Here we have a media query that says apply any classes listed out inside of the brackets, only if the users' screen size is more than 640px wide. If you wanted a class that affected only your mobile view, you would write it outside of the media query's brackets, as the media query content is meant to override any similar mobile classes.