Normally, the blocks would fall under one another on a mobile device, so if you have a line of images and a second line of text descriptions for these images (typical for product display), then on a mobile device, the descriptions will separate from images.
Image/Text is a special type of content in our editor which allows you to tie text and image together so that they do not separate on responsive devices.
When you set the content type as Image/Text, the selected block will populate with two items. On the left you will be able to open the Gallery as outlined here. On the right you'll be able to click to add text as outlined here.