Blog hero vs preview image

What is the difference between these 2 images? I don’t think I want to have every single blog post have a giant hero image (it looks weird IMO) but if I don’t enter a hero image the preview display shows the default blog image.

What is the preview image used for? I would assume (and wish) the preview image is the thumbnail image that appears on other parts of the site like the features blog spot on the home page. Is it possible for me to have those rolls switched?