The Constitution is a document, a supreme law that specifies the rules and bases of a State. In other words, it is the most important law and all the laws of the State are subject to it.
The Constitution establishes the basic obligations, freedoms and rights of the citizens, how power is organised, how the powers are divided, how the territory is organised, how the institutions, etc. function. It also specifies rights and obligations and what resources the State needs to set up in order to put them into practice. The Constitution also specifies what the political structure of the State will be like and the basic procedures that other laws have to follow. As pointed out already, the Constitution is usually the supreme document specifying the juridical organisation of the State, it is the highest law.
As it is the main law governing the political, social, economic and other activities of a State, it is written down. It has two main parts: the part regulating organic relations or the ones between the powers, and the dogmatic part that establishes the rights and obligations of citizens.
In short, the Constitution controls and structures political power. It also recognises human rights. The Constitution does not in fact grant rights. These rights belong to human beings and they are above the power of the State. That way, the essential reason for the Constitution would be to guarantee the procedures so that human rights are respected.
Even though the first ones were written down in the Middle Ages, modern constitutions came with the liberal revolutions of the 18th century, in particular as a result of the independence of the United States and the French Revolution. Later on, constitutions were specified and developed all through the 19th century.
The next important moment for Constitutions came after the Second World War thanks to the Universal Declaration of Human Rights (1948). Since then, the constitutions of most countries have included human rights.
The first constitution of Spain was written in 1812. The last one, the one in force now, was approved in 1978. In France, the 5th Republic was approved in 1958 and with it the constitution currently in force in the country.