Field Mapping is a step of matching your feed file columns to your store's product fields.
On the left is the store's product fields and on the are the feed file columns.
What are the store's product fields?
These are the fields that show up in your Shopify store. How your products will display in your store? You can choose which fields to add/update by clicking on the field.
What are feed file columns?
These are the columns available in the feed file that is provided by your supplier or drop shipper. You will need to match these to the store's product fields. You need to get the value from the feed file. It could be either index form or value.
To know more about each field:
Field Mapping (Step 2)