What does skell mean used on Law and Order?
That is a slang word for someone who has commited a crime.
The phrase of 'law and order' refers to the sanctity of the courts and legislatures to social stability. The maintaining or 'order' is the discipline and procedure of the courts and congress for example. 'Law' in turn can reference the police procedure of implementing and executing the will of the courts and legislatures.
Law is needed in order to maintain a society, a state, and the country in its entirety. Without law there would be not regulation to peoples behaviour. Therefore to say that the primary purpose of the law is the maintenance of order is to say without it there would be no rules on how we should behave, everyone would do as they pleased and there would be no order in society.