It means that, if you have been employed by Qatar Airways before, this might be relevant to the application process.
To pull up the correct file on your previous employment with them, the airline is requesting some sort of employee number, which will lead them to details such as your length of employment, reason for leaving the company, any performance review you may have had, discipline record while with the company.
If any of the above sound like a concern to you, you may just want to pretend you are a brand new applicant that has never worked for Qatar Airlines.
However, if they find out - lying in an interview can also be seen as falsifying information, so the offer could e withdrawn even if you get the job,
Assess the risks and make the right call, that's all I can say buddy