A contactless debit card is a debit card with a contactless payment feature enabled within the card.
This means that you can pay for goods and services up to £100 with your debit card without having to enter your PIN. Simply tap your debit card (providing it is a contactless debit card) onto the merchant's payment terminal and payment will instantly be made.
As a security feature to protect you, your card and your money, the first card transaction you make will require you to input your PIN. You will also be required to input your PIN if you make 5 contactless payments using your debit card within a single day. This again is a security feature to make sure it's you using your card.