Test objects are the representative data sets your colleagues chose for you to test your work with. You can switch them around to insert different data in your translation items text and see how the translation holds up.
The respective dropdown, titled "Preview Test Object" is located on top of the screen, next to the search field, warnings and error messages.
Please keep in mind that those data sets are not necessarily overlapping with current productive data your colleagues are using, as they are snapshots from the time they were created. If the data was changed in the meantime, the test object is not updated automatically.s