
Click on the "Concatenate" box to define an aggregation field (the word "Concat" will be automatically added to the default value field and it will be disabled)
Click on the Insert button first to add a new record

Select in the drop down box the Source column you want to concatenate to the selected column
Fill the "Concatenation string" field with the SEPARATOR you want to use (example : space dash space ie " - ")
When this column is imported, it's value will be followed by the "Concatenation value" and then again the value found in the SECOND column you have selected here
The concatenations will be performed in the order defined here
Example : You can select column "Name" in the Destination list and here define the SECOND column as "First name" and using a separator of " - " obtain this result : "NAME - First Name"