This is a blog post excerpt, which is showing instead of the full body text of the post. This post also has a thumbnail image associated with it. In this template, if you set both of these, then they will both show on the main Blog Page.Read More
In this template (Five), you can display a thumbnail image next to the excerpt. Not all templates do that. You can see which ones do on my Squarespace template comparison chart (also located in my book). Even if your template doesn't do that, you can recreate a very similar layout using a Summary Block. Be sure to scan through all 8 posts in this sample Squarespace blog because each one shows a few different settings/options/content types.