A restricted key is one whose profile is controlled, so it cannot be reproduced freely just anywhere. Blanks with that profile are not sold on the open market, and new keys can only be ordered through the channels the manufacturer authorises.
It normally comes with a key security card. Without that card, nobody should be able to get a new key made for your cylinder, not even someone who has had your key in their hands for a while.
It is one of the advantages of high security lock cylinders, and it makes a lot of sense in rental properties, second homes and houses where cleaners, gardeners or neighbours have a key. That way you stay in control of how many keys to your door exist.