When applying for credit, what is a co-signer?

When you're approved for a credit card with a co-signer, you and the co-signer become joint account owners and are both responsible for repaying the balance.

This is what a co-signer does. A co-signer is a person who is obligated to pay back the loan just as you, the borrower, are obligated to pay. A co-signer could be your spouse, a parent, or a friend. The lender cannot require your spouse to be a co-signer unless you are both applying for the loan.