A very mild punishment.
This is used in the context when one receives not so severe punishment for the deeds he does.
1)I received a slap on the wrist when I did not obey my father's orders.
2)Rahul received a slap on the wrist when he opened his mother's purse without her permission.
3)Peter gave a slap on the wrist to his neighbour's daughter when she tried to sneak into his house without his knowledge.