The key security card is the card that comes with many high security lock cylinders and proves that you own that key system. It carries a code that identifies your cylinder.
To order new keys from the manufacturer or from an authorised outlet, you have to present it. Without it, nobody should be able to get a key made for your door, even if they have had yours in their hands.
Three simple tips: keep it at home, away from your key ring; do not hand it over with the keys to whoever looks after your property; and take a photo of it in case you lose it. If you let or sell the property, the card should change hands with it. It is one of the strongest reasons for fitting high security lock cylinders.