A data structure column represents a database table column, a flat file column, an XML element or attribute, or an output parameter of a web service operation at a particular point in the transformation.
For example, you may have a column called Name in your source database, which is extracted and processed by a number of transformation actions before being loaded into your target database. Each of these steps in the transformation task will contain a data structure column, which represents the column at that point in the transformation. The column may be renamed filtered, reordered, and/or have its data type, length, default value etc, changed, and you can trace each of these changes by referring to the data structure column at the relevant point in the transformation.
You can use the Data Structure Mapping Editor to show how source and target objects data structure columns are mapped. For more information, see Data Structure Mapping Editor.