Put simply, a student guarantor is someone who is prepared to take on the main responsibilities of your rental contract. In the case that you are unable to fulfil the responsibilities of your contract, a landlord will contact them. They will then have to meet the responsibilities of your contract. This will often include being responsible for any rent that is due.
You need one because, as a student, landlords have no real way of knowing if you can afford to pay. They can’t check your employment history as most students don’t have any. They also can’t check your history of living in the UK if you’re an international student. A guarantor can offer landlords peace of mind. They will have lived in the UK for a long time and will have the means to pay any rent in the event that you cannot.
Many international students worry about this. In some cases, landlords will ask for some or all of the money for the tenancy upfront. It is sometimes negotiable the amount you will have to pay upfront. If you are unable to do this, there are companies who are willing to act as your guarantor for you, for a fee. Do your research and look into reviews for such companies before you enter into any agreement with them and hand over any money.