A secured creditor is a creditor that if you did not pay them, they could take something away from you. A good example of that is a home mortgage creditor, or the bank that holds the note on a vehicle. An unsecured creditor is a creditor who would have to sue you before being able to take any of your property. A good example is a credit card company or an unpaid medical bill.
When you file a Chapter 7 or Chapter 13 bankruptcy then you can reaffirm the debt and continue to pay for a secured creditor if you want to keep the collateral. If you sign the reaffirmation agreement on a vehicle, then you are reaffirming to the creditor that you intend to keep the collateral and pay the debt.
At the same time, if you file bankruptcy because you have overwhelming credit card debt, medical bills or are facing repossession or foreclosure or some other related problem then you do not have to sign a reaffirmation agreement. If you wish to surrender a secured asset back to the creditor, then the debt should be discharged if you do not sign a reaffirmation agreement.