A government of laws, not of people: Under a government of people, whatever party holds the reigns of government, has as a guide for its conduct only its own judgement, wishes, or whims. Whatever it dictates, is law.
Under a government of laws, the party that is in power must obey the laws that exist, the same as the rest of the people, and the laws must be passed by a prescribed procedure: if the parties not in power can hold it (the party in power) to the deal.
The framers of our Constitution tried to ensure that our government would be a government of laws. First of all, they made a written constitution. Having our principles of government in written form holds always before the party in power a sort of moral restraint concerning what it is supposed to do and what it is not supposed to de. Having a written constitution also serves the parties not in power (including the common citizens) in that it gives them something to hold forth as an example of how they should be treated. If they appeal to either their governors, public opinion, or the courts, they can cite principles in the written constitution. The Bill of Rights of our Constitution is very important from the standpoint of always keeping before both the rulers and the ruled, the principles of our government.
Second, the framers provided that the three branches of our government (executive, legislative, and judicial) would be separate in that no one person may serve simultaneously in two or more of the branches. Therefore, each branch serves to check the activities of the other two. This serves as a curb on serious abuses of governmental power. Also the powers given to each branch do not much overlap. The judiciary can only judge; it cannot legislate or execute. The legislature can only legislate; it can neither judge nor execute (except that in Reconstruction, it did). The executive cannot judge, but does participate in the legislative process, though never independently, always with the legislature (if you don't count executive orders, but how can you not count them). This is not as good of a curb as it might be, because all three branches are chosen by the same constituency, the democratic majority. If each were chosen by separate constituencies, such as wealth, population, and education, the separation of governmental branches would be a better protection of the citizens' rights.
Under a government of laws, the citizens have a pretty good idea how they will be treated from day to day and in response to whatever action they each or in groups might perform. the laws can be read and understood. Under a government of people, the citizens are at the mercy of governors who are not guided by laws and so may treat the citizens arbitratily; the citizens do not know what to expect; life is uncertain for them; is it safe to enter into a business, or will the governors decide they don't like my type of business? Will the governors give a monopoly of all of my type of business to their special friends? And so on.