When you select Google from the channels list you will be prompted to enter the website URL that you claimed in your Google Merchant Center.
According to the Google Shopping Policy, the domain in the product URLs (links) must match the domain claimed in the GMC.
DataFeedWatch compares those two domains. If they are different it will automatically replace the domain in product URLs (links) with the domain provided in this field.
You can edit it later on the map fields page.