This allows you to lock any car. The car keys come in several colors and also have tags for easier organisation. You can buy the keys separately at trader.
Take key in hand, look at car (must have all doors closed and all doors including trunk and hood in the slots). It will give you an option to assign key to car. After assigning key you can use it to lock, unlock the car.
On all car traders you will found some grinder around where you can duplicate your key. You can only make duplicates if key was assigned. Your key takes damage each time you make a copy. e.g. 1 key gives you 2 keys before its ruined. then you can turn 2 keys into 4 and so on.