In the broadest sense, police power is what allows a government to create and enforce the law. In America, it allows the states to create laws and rules in order to protect their citizens. The Tenth Amendment of the Constitution bestows police power to the states, which in turn share it with the smaller government entities within their borders.
States exercise their police power by creating state laws or statutes. In certain localities, the police power conferred by the state allows governing bodies to create zoning ordinances, nuisance laws and parking regulations.
Although it is fairly broad, there are some limits on police power. Specifically, laws based on police power must be created for the good of the community. Laws that exceed the scope of this authority are often challenged if they unduly restrict freedom or restrict access to or the use of someone's property.