At the bottom left of your site, when you are logged in and in Edit Mode, there are three buttons to determine which “Viewport” you are working with. One is for desktop/laptop, one is for tablet, and the other is for cell phone/mobile devices.
These buttons are useful for two things:
To see what your site looks like in different viewports
If you click on a viewport and then choose “Arrange Sections,” you can move items around and create a separate section arrangement for each viewport type. This can be useful for pages with a lot of sections where the default arrangement doesn’t look as you would like it to on mobile or tablets.
NOTE: Changing viewports does not give you the ability to enter separate content for each viewport type, only a separate section arrangement using the Arrange Sections option.