Wikipedia
Rechtsstaat is a doctrine in continental European legal thinking, originating in German jurisprudence, that can be translated as "legal state", "state of law", "state of justice", "state of rights", or "state based on justice and integrity".
A Rechtsstaat is a " constitutional state" in which the exercise of governmental power is constrained by the law, and is often tied to the Anglo-American concept of the rule of law, but differs from it in that it also emphasizes what is just (i.e., a concept of moral rightness based on ethics, rationality, law, natural law, religion, or equity). Thus it is the opposite of Obrigkeitsstaat (a state based on the arbitrary use of power).
In a Rechtsstaat, the power of the state is limited in order to protect citizens from the arbitrary exercise of authority. The citizens share legally based civil liberties and can use the courts. A country cannot be a liberal democracy without being a Rechtsstaat.