Constitution is the stat in D&D which determines how many hit points your character has... though in legal circles it can also mean the basic governing law of a state, which sets out (usually, though not always, in the form of a single document) the principles by which the state's government is organized, who is subject to it, and what the laws of the state can and cannot govern. Most countries have one, though some states, such as the United Kingdom, New Zealand and Israel, have no specific, entrenched constitutional document, relying instead on existing legislation, case law, and the doctrine of parliamentary supremacy as the basis for the rule of law. In these countries, legislation and case law relating to government powers and the rule of law are loosely referred to as constitutional law.
Though the term seems to mostly be a product of late medieval and early modern jurisprudence, the elements of constitutions go as far back as early law codes such as the Twelve Tables of Rome and the Hammurabi Code of Babylonia. Modern examples range from the relatively brief constitutions of the United States or Germany, to the extensive proposed European Union constitution, to the somewhat nebulous common law-based body of legislation that makes up the British constitution. In the countries with a brief constitution, the text is really vague, so they have a Supreme Court that makes up the rules as they go along – kind of like when your big brother taught you how to play Monopoly when you were a kid.
In the United States, when liberals and moderates are appointed to federal jobs, they put their hand on the Bible and swear to uphold the Constitution. When conservatives are appointed to such posts, they put their hand on the Constitution and swear to uphold the Bible.
Magna Carta (which was not 'signed' by King John, and which was only in operation for a few weeks) and its successors were not a constitution in the modern sense, though they are considered to have formed the beginnings of the British Constitution.