This means that the given image will be stretched to the full width of the mobile screen.
It appears for images that are wider than 240 px, but are scaled down below the 240 px limit.
If you upload a big logo.png, but you resize it to 200px in the editor with the image slider, the fluid on mobile switch will show up
When it is switched to "ON", your logo.png will be stretched to the full-width of the mobile screen; if it's "OFF", it will stay 200px wide
Fluid on mobile switch off
Fluid on mobile switch on
Here is where you can find that feature:
If you have more questions please contact us via chat or email to firstname.lastname@example.org