This option allows you to set a risk payment threshold in case of payments made with non 3D Secure enrolled cards that are fraudulently used. Basically it means that if a buyer uses (fraudulently or not) a non 3DS card and the payment is higher than the threshold you set for a certain currency then the payment is declined.
From the merchant perspective the usage of this option should be seen as a compromise between shifting the liability towards the bank while losing some potential customers due to non 3DS enrolled cards use and selling to these potential customers while assuming a certain liability within a controlled risk range.
You are allowed to set a single threshold value per currency, ie. if you use 10 different currencies in your web shop you may set up to 10 different payment risk thresholds, one for each currency. When no threshold (or zero) is set for a currency it is assumed that you don’t accept any liability for orders made with that currency.