A guarantor doesn’t have to be a parent but usually is. A guarantor takes on some of the risk of you being unable to meet your repayments. The lender will normally require your guarantors to offer their property as security against the guaranteed part of the mortgage.
Technically they become immediately liable to repay the outstanding loan if you are no longer able to make your payments. In reality what usually happens is an agreement is made between the lender and the guarantor, so they maintain payments until you are able to do so.
The amount of lenders willing to consider this for buy-to-lets is very limited.