"From the production itself we are importing again.
I think by using menubar -> import we are re import the Request again."
This does not apply only to production system it can be QA also, process is same for all systems depends to the requirements where you want to move "again".
"I was thought re import means again create one more tr and we are importing it in production server with in all ready imported tr(Yellow color tr number)"
This comes when you want to add or update the existing objects and save it it will generate a new transport which you can move into another systems for that additional changes.
Regards,
Subhash