This is an instance of the image gallery plugin
Photo galleries are recommended when images are the main content of your post. When you use the photo gallery post type, the images added to the post will automatically display in a gallery format. The teaser will display an icon that symbolizes a photo gallery, but visitors will only be able to see the gallery when they click through to the full post view.
Photo galleries can also be embedded within a post so that it appears between paragraphs of text. See Advanced configuration settings for images to learn how to embed photo galleries.