My car loan has been handed over to an underwriter, what does that mean?

I traded in my 03 Cavalier, or am trying to trade it in for a 08 Caliber. I signed my half of the paperwork that night, however my co-signer resides about an hr away and hasn't signed his half. The paperwork is not complete and the loan has been handed over to an underwriter. However they allowed me to drive of the lot in the new car. At the time of signing we presented the co signers license, last three yrs of W-2's, and a current check stub, along with my check stub. It worries me that the loan wont go through and they will take the car back that they have allowed me to drive for a week now. Any additional info would be greatly appreciated.