The landlord is entitled to demand that all adults over the age of 18 co-sign the lease jointly and severally liable. This means that if any one of the co-tenants does not pay their portion of the rent, all the tenants become liable for any outstanding portion.
It is important that you, the tenant, understands your responsibility to the landlord. If you are unsure of your co-tenant's ability to timeously pay their portion of the rent, it could affect your continued occupation of the property and your credit profile.