Metafields allow you to add additional custom data in your shopify store. Metafields can be used in products , variants, collections, customers, orders, page, blogs, and articles. Metafields Editor allows you to create, add, and modify metafields in your store. The data itself is stored by Shopify and not by the application. Metafields remain with your store even if you uninstall Metafields Editor app.

• Metafields is used to describe additional attributes about your products , variants and collection etc. They are often used in Shopify theme development to store product information that cannot be stored in Shopify Admin. Theme designers utilize metafields to store information such as additional attributes on products and variants, size charts, image, video information and much more.
• You can add unlimited number of metafields for all resource types.
• The interface is very simple and you can perform almost all tasks on a single page. You can add new metafields, modify existing ones, and delete metafields.
• Please note that Metafields is a Shopify feature and it has nothing to do with SEO. Do not confuse it with meta tags for html.
• Metafields are added to Shopify via Shopify API.

Update Jan 2019.

Now you can do bulk import/exports by csv. Only charged for each record imported/exports and no need to pay for a monthly fee. With bulk csv import/export, you can modify metafields for product, variances, collections, customers, orders , blogs and pages.