The term 'risk' refers to the risk of loss if property is damaged or destroyed. The general rule is that risk of damage or loss falls on the owner of the property, unless some other rule shifts the loss to some other person.
'Frustration' refers to the situation where, because circumstances change after a contract is entered into, performance becomes impossible, either at all, or in the way envisaged when the contract was made. If, in the circumstances of the case, the changed circumstances are held to frustrate a contract, this will discharge any outstanding obligations from the moment of the frustrating event.