Summary:
There are two type of data that can be imported into Web TimeSheet they are, Project related and User related.
In the CSV file the Key column identifies to what user or project the newly added data should be assigned, or what user or project's data should be updated. For User imports, the Key column is Login Name. For Project imports, the Key column is Project Name.

Note:
While creating the CSV file make sure there is a key field. The key field for each type of import is always required, since that is the field Web TimeSheet uses to match new and existing data. Other fields may also be required, depending on what you are importing. The key fields are entered in the key column.

Refer to the attached screenshot: