What a constitution does
A constitution establishes how a country is governed -- defining the structure of government, the powers and limits of each branch, the relationship between different levels of government, and (usually) the rights guaranteed to citizens. It serves as the supreme law, meaning ordinary laws that conflict with it can be struck down.
Types of constitutions
A written constitution exists as a single, formal document (like Nigeria's or the USA's); an unwritten constitution is built from various sources -- statutes, conventions, court decisions -- without one consolidated document (like the UK's). A rigid constitution requires a special, more difficult procedure to amend (often a supermajority vote or referendum); a flexible constitution can be changed through the ordinary law-making process.
Example 1: Nigeria's 1999 Constitution requires approval by two-thirds of both houses of the National Assembly and a majority of State Houses of Assembly to amend it. Is this a rigid or flexible constitution?
This is a rigid constitution -- amending it requires a special, demanding procedure well beyond passing an ordinary law, deliberately making changes harder than routine legislation.
Rule of law and constitutionalism
The rule of law means no one, including government officials, is above the law -- everyone is equally subject to it and accountable through the courts. Constitutionalism is the broader idea that government power should be limited and exercised according to established constitutional rules, not the arbitrary will of whoever holds power.
Fundamental human rights
Most constitutions guarantee certain basic rights to citizens, such as freedom of speech, freedom of association, the right to a fair hearing, and freedom from discrimination. These rights limit what government can legally do to individuals, and citizens can typically challenge violations of these rights in court.