It implies that the consequences for a wrongdoing are minimal and don't have a significant impact.
Here are some examples:
* "The company got a slap on the wrist for polluting the river." This means the company was punished for polluting, but the punishment was not severe.
* "The child got a slap on the wrist for breaking the vase." This means the child was reprimanded for breaking the vase, but the punishment was not harsh.
The idiom comes from the idea that a physical slap on the wrist is a relatively minor form of punishment.