Here you're telling the browser that if the viewport width is wider than 75em it must use the large image. Between 40em and 75em the browser must use the medium image. Below 40em the browser must use the small image.
Here you're telling the browser that if the viewport width is wider than 75em it must use the large image. Between 40em and 75em the browser must use the medium image. Below 40em the browser must use the small image.