Result grouping enables you to remove duplicated search results which may occur due to multiple child products that were imported to the Nosto database. If these child products have a parent ID field it is possible to collapse the child variants to the parent ID - for each of the collapsed fields (e.g. parent_ID) the search core will return only the most relevant product.
Please note that the fields that you want to choose for collapsing, have to be added to the indexed fields beforehand.
Please use this feature with caution, as you won't get all products of a group and it might happen that your are not able to show all available colours or sizes.