This is best explained with an example.
Below, Conga Mail Merge is expecting the first value to be "ID" rather than "Product__c.ID".
SELECT Product__c.ID Product__c.Name, Product__c.Type, FROM Purchase_Order_Product__c WHERE Product__c.Vendor='001T000000mABSn'
This error would be corrected by using the following:
SELECT ID Product__c.Nam FROM Purchase_Order_Product__ WHERE Product__c.Vendor='001T000000mABSn'
This formal naming convention became popular after the introduction of Conga Mail Merge.