Advanced Custom Fields allows you to build custom fields to a Custom Post Type. If you were creating a site selling vehicles and wanted custom fields to integrate car type, color, size, model, brand, etc., you can create those fields and assign them to the post types so that every time you add content, the backend is modeled to ensure you load all necessary content.
This introduction not only covers ACF, but also covers using “Ele Custom Skin” to create custom loops. Loops are the post cards you see on the right side of this page. Every time a post appears as a list, it’s called a “loop.” You can use ACF without building loops — wptuts brings them together because understanding ACF is important to understanding loops. For more on loop plugins, visit the Post Loops post.