The constitution is the fundamental law of a country, and it defines who, what and how the country is and should be. In other words: What area of land is covered by the constitution, what kind of government rules, and what are the basic rules that apply within the state.
A country may have a constitution without being a democracy - an example of such a constitution might be one stating that this is an absolute monarchy, or that the president has full sovereignty. But very few democracies do not have a constitution. The UK is one such example of a democracy without a constitution. Instead, the UK has a doctrine of the sovereignty of Parliament.
Typically, the constitution of a democracy explicitly states that the power lies with the people. One of the clearest democratic constitutions is the Swedish constitution, which opens with those exact words: "All public power in Sweden lies with the people."
All laws of a country must comply with its constitution, and the constitution defines how new laws are to be passed.
The constitution also includes rules on how the constitution itself may be changed. Because the constitution is essential to all other laws, usually very strict rules apply as to how to modify the constitution itself. In Denmark, a constitutional amendment or change requires that once the parliament has passed the constitutional change, there must be a general election and afterwards the new parliament must also pass the constitutional change. When the change has been passed a second time, it must be put to referendum, and not only must there be a majority in favour of it, a minimum of 40% of all eligible voters in favour of the amendment or the change is also required.
Next chapter: A Legislature Elected by the People