The sidebar uses position:fixed;
to stay where it is. The responsive side (re-sizing the browser) is most likely the use of media queries. Example:
@media only screen and (max-width: 999px){
//if screen is less than or equal to 999px (just a random number) execute this code
}
@media only screen and (min-width: 1000px){
//if screen is greater than or equal to 1000px (just a random number) execute this code
}
UPDATE