Peelian principles are some important rules made by a man named Sir Robert Peel. He made these rules a long time ago to help the police do their job in a fair and helpful way.
One of the most important Peelian principles is called "The basic mission for which the police exist is to prevent crime and disorder." This means that policemen and policewomen are here to help people and make sure bad things don't happen. They don't just show up when something bad happens.
Another idea that Peel came up with is "The test of police efficiency is the absence of crime and disorder, not the visible evidence of police action in dealing with it." This means that the best way to measure how well the police are doing is by how much crime and trouble there is. If there's no crime or disorder, then the police are doing a really good job.
Peelian principles also say that the police should be honest and fair with everybody, no matter who they are – rich or poor, young or old, male or female. This is very important, because it means that everyone can trust the police and feel comfortable talking to them if they need help.
Overall, Peelian principles are just a few really important rules that help make sure that the police are doing their job in a fair and helpful way.