Using the Map column option, you can map an attribute's value from your source store to another attribute on a target channel.
The value of the given field will be copied from the source field to the target field.
To map a given field using Map column, follow the simple steps below:
Find the field that you would like to map.
From the first drop-down list next to it, select Map column.
In the next field select the target channel field that you would like to map into.
Choose if you'd like to use this field for All Products, or add an If condition.
Map the source SKU to the ID on Google Merchant
In the example above, Google's field ID with the mapping type Map column for sku will copy your source SKU into the ID field on Google. This way, the requirements of Google will be met, and the field id required by it will be completed with sku values from your source.