Idiom: A slap on the wrist

Examples: 1. The murderer got "a slap on the wrist"--- only three years in jail. 2. The teenager hit his teacher but only got "a slap on the wrist"--- a two day suspension. 3. The high school student cheated on his history test, but his teacher just told him not to do it anymore. He barely a "slap on the wrist."

What is "a slap on the wrist"?

It's a very light punishment compared to what most people get for doing the same thing wrong.