Click on All screen sizes to unfold a menu with different screen sizes. By clicking on each screen size, you can see what your website looks like in that view.
XS screens are extra small mobile views (e.g. iPhone 5)
SM (small) and MD (medium) screens are mostly mobile as well, like iPhone 6 and higher
LG (large) screens are the size of most tablets
XL screens are laptop and desktop screens
Every change you make only applies to the view that’s selected. This way you can make specific changes for, let’s say, your mobile website.