Criminal liability of a police officer can be defined as the fact that some actions that a police officer can take are crimes. A police officer who takes such an action can be charged with a crime.
Perhaps the most high-profile example of such things in recent times is the case of the police officers in New Orleans following Hurricane Katrina. In this case, some police officers were found guilty on a variety of charges because they shot some civilians without cause and then took various actions to cover up what they had done.
Criminal liability is the idea that actions such as this can be prosecuted as crimes.