Metafields mapping is a new feature that has been added to Google Shopping Feed by AdNabu.
Using Metafield mapping, you can map any metafield from Shopify to any other attribute in Google Merchant Center. By default SKU in Shopify is used as MPN in Google merchant center. You can override this setting by using Metafield:
Step 1: Go to Settings>Feed.
Step 2: Inside Feed, you will be able to see a new segment called Product Metafields:
Step 3: You can use any Metafield App to create a new Metafield for the products.
Step 4: Then, you can map those metafields created using the app, to the attributes in Google Merchant Center.
For more details, you can have a look at the video.