A co-signer lease agreement is a legal contract between a landlord and tenant, where a third party agrees to be liable for the rent payments if the tenant fails to pay. This form of agreement provides an additional layer of security and protection for the landlord and ensures that they are still able to get their money, even if the tenant defaults on their payments. It is a great way to make leasing more secure, while guaranteeing that everyone involved feels safe and protected in their relationship.