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.
This is a blog post without an excerpt. (Notice there is no Read More link). If you don't include an excerpt, then the entire text of the post will appear in the main Blog Page. This is how blogs used to work way back in the 90s, and many people still prefer this approach to blogs because it means people can see all of the content without needing to do anything but scroll.
However, some people prefer to have an excerpt instead, because it means that you can fit more posts on a single screen without requiring scrolling. Plus, it can look nice and tidy if you always use an excerpt (and thumbnail image).
This post actually has a thumbnail image set, but you can't see it when looking at the main Blog Page, or at the post itself. The only time you will see it is if this post appears in a Summary Block. Summary Blocks always look nicest if you include a thumbnail image.