To add a comment to a field, open the tools menu for the field by clicking on the cogwheel to the right side in the field:

A dialog window will appear in which you can enter your comment. It is possible to make multiple comments however, it is important to note that comments cannot be deleted or edited after they have been added. When exporting the study data in csv or Excel formats, comments will be exported in a separate field. Please note - comments are not exported with the SPSS export option.

Example of the comments export in Excel/CSV format: