When a Landlord agrees to accept a tenant into his or her property, they are taking a risk that the tenant will pay the rent for the duration of the tenancy period, look after the property, and move-out when agreed. They want to take steps to minimise the risk they are taking. Tenants who are in full-time employment perform credit and referencing checks to check affordability. This is not suitable for some tenants, i.e. students. In those cases, a Guarantor is required. A Guarantor is one way of a prospective tenant demonstrating their suitability and securing the property.
Where the tenant cannot provide a guarantor they can either pay the rent in advance or use a guarantor service such as https://www.housinghand.co.uk/