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.

This blog post has a full-width image at the top of the post. Sometimes this can mean lots of scrolling before you see content. It also has a video in it, too.

This post shows the different ways you can crop images in Squarespace.

Here's an example of how you can use a Summary Block to show a Related Posts list at the bottom of your blog posts.

Not all templates use the Promoted Block feature. The ones that do, show the first image in a special way that makes it stand out.