A co-signer is someone who legally volunteers their credit and finances to supplement another's lease when the applicant is unable to prove their own credit standing in a manner sufficient enough to be accepted on its own.
Co-signers are not involved in the payment of bills, however, unless the primary borrower fails to do their part.
Anyone can be a co-signer, as long as they're able to legally borrow money.
The specific requirements to be a co-signer will vary from one property owner to the next and will also depend on the amount being requested. However, as a general rule, co-signers must have at least "good" credit, roughly above 670. Your eligibility as a co-signer will also be reliant upon your amount of disposable income.
If you're unable to be accepted for a rental that you want to live in without having a co-signer, you're faced with a difficult choice: find a different rental or ask someone to co-sign the lease with you.
When considering a co-signer for a lease, you probably understand the potential for impact on the co-signer's credit report. Luckily, as long as you're able to pay your rent on time and never miss a payment, there will be no impact to the co-signer at all.
The only time the co-signer will be responsible is if you're unable to complete the terms of the lease.