Impartial and Independent Courts
For the laws of an elected parliament to be meaningful, there must be someone who ensures that the laws are respected. This is the task of the courts: the judiciary power.
Not only must the ordinary citizen obey the law, the law applies to the members of parliament and the members of the government as well. The latter is really a democratic litmus test! In most countries, members of parliament and the government enjoy immunity as long as they are in parliament and/or the government, but that does not mean that they are above the law. It means that they must be expelled from the parliament/government before they can be held liable for breaking the law.
The possibility of holding even members of parliament and the government liable for their actions is one of the reasons why it is important that the courts are independent of the legislative and executive powers, in keeping with Montesquieu's tripartition of power (see: A Legislature Elected by the People).
Next chapter: The Prosecution and the Police