The report preview lets you see how your finished report will look and gives you tools to change the viewer size, for example on a mobile phone screen. This can help you when using responsive design. You can access this preview via the Styling menu > Mobile View > Preview on the report editor screen.
You can either drag the grey box in the middle to a suitable size or use the buttons on the right hand side to choose some common screen sizes. The preview also shows a dotted red line at your current “switch size” (where your report starts to change to a responsive design).
Note that this is not a fully featured mobile phone emulator – it is designed only to give you an idea of how your report will look, it will not, for example, mimic using touch gestures rather than a mouse or zooming in and out on the screen. If you need that level of testing most modern browsers will let you switch views via their developer tools – for examples see Simulate Mobile Devices in Chrome and Responsive Design Mode in Firefox.