What is "due process of law"?

The Fifth Amendment's words that no person shall "be deprived of life, liberty, or property without due process of law" expresses one of the most important principles of the Constitution. The Founding Fathers shaped this constitutional guarantee because they believed that a person's life, liberty, and property should not be threatened by or taken away because of the arbitrary or unreasonable action of the government, but rather could only be limited if the government followed a proper course of legal action. The Fourth, Fifth, Sixth, and Eighth Amendments provide a number of procedural guarantees for those accused of crimes and, taken together, those guarantees are often called due process rights. The Fifth Amendment's same words are in the Fourteenth Amendment, which holds the states to the same restrictions.

reference: page 375, The Handy Politics Answer Book by Gina Misiroglu