Think of Fields as containers where you can store data about your products to enrich your product details.
To create a new field in Shoppingfeed, access your Catalog page and then click on Create new fields tab.
Create a name for your new field and click on Save Changes.
Note: If you want the field to have a default/static value, set it in the 'Current value' box. This means that the value you put down will fill that field by default.