A media query is an HTML/CSS functionality that allows the content of a Web page to adapt to the type of media that the page is being rendered in, such as a computer screen or that of a phone or tablet. This is considered as a core technology for implementing responsive Web design and was recommended for implementation as a standard in June of 2012 together with other CSS3 functionalities.
Techopedia explains Media Query
Media queries consist of a media type along with one or more expressions that conditionally check for certain media features, particularly that of screen sizes. The logical expressions in a media query can be either true or false; it is true if the media type of the query matches that of the media type of the device where the user agent (Web browser) is running on; otherwise, it is false. When the media query results to true, then the corresponding style rules specified will be applied, following normal cascading rules. It must be noted that, even if the query results to false, style sheets specified within the <link> tag are still downloaded, but are simply not applied.