When you were a kid, you were always punished for doing something wrong. Sometimes the punishment was not as terrible as you had feared; in fact, it was rather moderate. When you are slapped on the wrist for doing something wrong, your punishment is less severe than it could have been.
Those who drive without a license should not get off with a slap on the wrist. They should be imprisoned.

The kids were given a slap on the wrist for failing to complete their schoolwork.

The phrase 'get your wrist slapped' means the same as 'slap on the wrist.

We were slapped on the wrists for not drinking the milk.

When you are proud of what someone has done, you usually slap him or her on the back. The expression "slap on the back" is used to express appreciation. It has the same meaning as 'pat on the back'. Rahul deserves a pat on the back for his assistance in organizing the cultural event.