What does the css media query function do code example

Example: css media queries

@media only screen and (max-width: 1200px){
    /*Tablets [601px -> 1200px]*/
}
@media only screen and (max-width: 600px){
	/*Big smartphones [426px -> 600px]*/
}
@media only screen and (max-width: 425px){
	/*Small smartphones [325px -> 425px]*/
}