A doorknob is an item used in carpentry.
The only purpose of the doorknob is to craft doors and drawers.
If the player uses the doorknob from a door they have the key to, crafting a new door can also be locked/unlocked with the same key.
It is found mainly in the hardware store, but a destroyed door will always yield a doorknob, along with some planks and some door hinges.
From items.txt (Project Zomboid directory/media/scripts/)
DisplayCategory = Material,
Weight = 0.5,
Type = Normal,
DisplayName = Doorknob,
Icon = DoorKnob,
MetalValue = 10,
WorldStaticModel = DoorKnob,