Summary
After editing your theme and previewing it across different devices you may find that some sections just look better on some devices. For example on mobile devices you may want to use a slider instead of a grid section.
How to
Open the theme editor by clicking Customize next to the relevant theme.
Create two sections, one will be on desktop and one will be for mobile.
Click into one of the sections and scroll down to Display then click Show on mobile only.
Click into one of the sections and scroll down to Display then click Show on desktop only.
Thats it! Now you have two sections that each only display on mobile and desktop separately.