Preventive police is when police officers try to prevent bad things from happening before they actually happen.
Think about it like being a superhero - but instead of waiting for something bad to happen and then rushing in to save the day, preventive police officers try to stop the bad thing from happening in the first place.
For example, let's say the police hear that some people might be planning to break into a house. The preventive police officers would try to stop that from happening by being in the area, making sure the house is secure, and even talking to people they think might be involved to convince them not to go through with it.
The idea behind preventive police is that if you can stop bad things from happening before they actually happen, everyone will be safer and there will be less damage and harm to people and property.