What is the rule of law

We, human beings are social animals. This means that we are highly interactive with other members of our species and we have clearly recognizable and distinct society.

Social individuals need to cooperate and cooperation requires an organization. Decisions are more efficiently taken by one or a few members of the group, the necessity of a leader is relevant. Cooperation and leadership require a set of rules and there is where the law comes into effect.

Early groups of hunter-gatherers had chieftains which later developed into monarchs. The monarch was probably the strongest or the wiser. The monarch imposed his rule. Religion helped to attribute legitimacy to the monarch as getting his power from God.

Democracy (government of the people) is based on popular sovereignty. The power doesn’t come from the divinity but from the people. The people, through its representatives, elaborate the laws governing the society.

Rule of law implies that every citizen is subject to the law. It stands in contrast to the idea that the ruler is above the law, for example by divine right (a monarch is subject to no earthly authority, deriving his right to rule directly from the will of God).

Aristotle declared, "The rule of law is better than the rule of any individual”.

Legitimacy is the popular acceptance of a governing law or regime as an authority. 

  The political legitimacy of a civil government derives from agreement among the autonomous constituent institutions —legislative, judicial, executive — combined for the national common good; legitimate government office as a public trust, is expressed by means of public elections.

Charismatic authority derived from the leader’s charisma, based upon the perception that he or she possesses supernatural attributes, e.g. a clan chieftain, a priestess, or an ayatollah.

Traditional authority derived from tradition, wherein the governed populace accepts that form of government as legitimate because of its longevity by customs, e.g. monarchy.

Rational–legal authority derived from the popular perception that the government's power derives from established law and custom (a political constitution), e.g. representative democracy.

the central institutions for interpreting and creating law are the three main branches of government, namely an impartial judiciary, a democratic legislature, and an accountable executive. 

Democracy is generally defined as a form of government in which all adult citizens have an equal say in the decisions that affect their lives.

Many people use the term "democracy" as shorthand for liberal democracy, which may include elements such as political pluralism; equality before the law; the right to petition elected officials for redress of grievances; due process; civil liberties; human rights; and elements of civil society outside the government.

A lone Robinson Crusoe doesn’t need any law. The necessity of rules stems from the organisation of human beings into societies.

Small groups are the early organisation of humans. The bigger the group, the greater the necessity of a set of rules.