This means, that you can upload an image that will appear specifically on mobile. This is a great option to use if you find that you don't like how your main desktop image for that section appears on mobile, or just want to change it up!
Keep in mind, that these sections also include an image cropping setting on mobile as well. If you'd like to allow a greater display area on mobile, you can enable 'image cropping' which will crop and zoom in on the image (left, center, right).
So no matter what, you'll be able to get your Image with Text Overlay and Slideshow images looking just right!