Deferred payments are essentially payments whereby the Card details are provided when the Order is placed, but the Card is not charged until a later date, such as the Delivery Date.
This makes reconciling payments easier.
To enable deferred payments, you must first enter your Sage Pay account details in Setup > Online Ordering > Payment > Sage Pay
Then, tick the box to 'Enable pre-authorization'
Following this, you should call Sage Pay to request that they update your Sage Pay account to allow capturing to take place.