In loans, a co-signer guarantees repayment because the lender believes the main signer has an unacceptably high possibility of defaulting on the loan. Use with the [loans] tag.
A co-signer is someone who signs for loans because the person who is requesting the loan is considered an unacceptable risk by themselves. This is typically based on credit-score, where the co-signer has a much higher score than the main signer.
This incurs some risk for the co-signer because if the main signer defaults, the co-signer is now legally obliged to repay the loan.