In most CMS system today, it's easy to make a website responsive, but since there are so many different devices to choose from today, it often needs to be customized a little extra, so that the design doesn't break up on some screen sizes.
Then it is possible to enter Viewports for the skerm widths not covered by the CMS system.
A Viewport defines which code should be applicable to a screen width of a span from so and so many pixel to so and so many pixel, eg: from 1600px to 1200 px. For example, the menu at the top of the webpage can be displayed with the tabs/menu buttons next to each other down to a certain width, before there is no longer room for all tabs next to each rower, and the design switch.
Then you can either change the e.g. font size, or change the menu to become a dropdown menu. In order for these changes to apply only from where the design breaks and on narrower screens, but not in the wide range where it can be displayed at full width, a viewport can only be entered for narrower skerm widths.