Hey Chris,
the CSS above makes use of the so-called media queries (the first line), which tell the system to apply the styles only given certain width of the screen of the viewing device currently used by the visitor. The second line acts just like in the previous snipped I’ve sent you, telling the background slider area to be 100px in height.
You can find some more information about CSS media queries e.g. here.
Sincerely,
Andrii / Satori Studio