A police officer is supposed to exert the least amount of force necessary to subdue a suspect. As soon as a police officer exerts unnecessary force, it may be considered police brutality.
The issue at hand is that the amount of force necessary can be subjective, meaning that not everyone will agree on what is necessary to subdue a suspect. Some cases of police brutality are clear, especially if they have been caught on camera. We are seeing more cases of police brutality in the news because more and more police officers are wearing body cameras that record every interaction with a suspect.