If your landlord or letting agent requires a guarantor, that person is signing a legally binding agreement to cover your rent and any other costs you owe under the tenancy (including potential deposit shortfalls or damage) if you fail to pay. This is a serious commitment, and your guarantor should understand exactly what they're agreeing to before signing.
Guarantors are most commonly required for students, tenants on benefits, tenants without a UK credit history, and tenants whose income doesn't meet the landlord's affordability criteria. The guarantor must usually be a UK resident with sufficient income to cover your rent if needed.
If you fall behind on rent or cause damage that exceeds your deposit, the landlord can pursue your guarantor for the money owed. This can affect your guarantor's credit rating and financial position.