Metafield definitions

Metafield definitions enable you to create additional validation constraints for metafields, and enable the merchant to edit metafield values in context. Where possible, you should adopt standard metafields, so that your app can access information that merchants have already stored in standard metafields. You can also create your own custom metafield definitions.

If you want to create and manage metafield definitions with your app, then you can use the GraphQL Admin API as described on this page. If you're helping a merchant to build their store, and you want to set up metafields in the Shopify admin, then refer to the merchant documentation about metafields.