From Car Loan Wiki

A Co-Signer is the person who signs there name on a legal contract agreeing to pay the debt of the original signer if they are unable to do so themselves. This is especially important for people with bad credit that are trying to purchase a vehicle, or people that are younger and most likely haven't established enough of a history of credit. Remember if the borrower cannot make a payment on an auto loan it is the legal responsibility of the co-signer to pay the debt. Failure of the co-signer to pay the debt can result in the co-signers credit being tarnished.