This is a post with post format gallery and the [gallery] shortcode inserted into the body of the post. When you set the post format to gallery, the theme will automatically take the images of that gallery and show them in a slider at the top of the content.
This is an image post. There’s isn’t really anthing special about it, other than selecting the “Image” post format. We’ve still just applied a featured image and set it to link to its enlarged version in a lightbox.