A guarantor is someone agreeing to take responsibility for financial liabilities such as rent if you default on it. This is usually a family member who is a home owner and/or in full time employment. The landlord might ask to carry out some checks on the guarantor's financial situation. If you can’t provide a guarantor you might be asked to pay the full year's rent up front.
Many landlords will carry out some sort of reference checks on prospective tenants. This can take the form of taking a reference from a previous landlord or University Halls, a credit check; or a reference from employer or place of study.