According to ApartmentRatings.com, a guarantor on a lease is a person who agrees to make the payments if the tenant becomes unable to pay the rent. The lease guarantor provides added security for the landlord as the guarantor is also responsible for any damages incurred.
ApartmentRatings.com says the guarantor of the lease signs the contract or lease, usually when the tenant's income is insufficient or credit score is too low to secure the contract on their own. According to About.com, guarantors are often the tenant's parents, who are usually in a stable financial position. A lease guarantor is sometimes called a co-signer by brokers and landlords.