The Discount DV01 result indicates how the instrument's market value changes for a 1 basis point shift upwards in the rates used to calculate the discount factors.
It is useful as it lets you see the effect on your market value of using different discount factors, i.e., discount factors calculated using the shifted underlying rates.
This result is only displayed for an instrument1 if you select either the regular cash collateralization method (where you can choose a collateral account currency that is different from the trade currency) or the variation of the cash collateralization method, known as CTD collateralization (which lets you select a number of collateral account currencies for the given instrument).
In addition, it is only displayed after you calculate the instrument.