Cankar
A product controller (also known as a BUC or business unit controller) is to make sure that the traders Daily PNL is calculated properly, is within risk limits and funding charges have been applied to correctly. You will be responsible for producing the daily PNL, checking the "mark to market" on a regular basis, explaining any differences to the trader where his/her PNL is different from the one you calculate etc.
You would also work closely with the Fin Reporting people on the balance sheet aspects of the trading book.
Obviously each bank's product controllers/BUCs have slightly different roles and responsibilites.
If you don't have the product knowledge then you should sweat up some product knowledge before the interviews.
Hope this helps - it has been years since I did product control.
Good luck !
Katanga