"No German may be deprived of his citizenship. Citizenship may be lost only pursuant to a law, and against the will of the person affected only if he does not become stateless as a result.-- No German may be extradited to a foreign country. A different regulation to cover extradition to a Member State of the European Union or to an international court of law may be laid down by law, provided that constitutional principles are observed."
Article 16 maintains that no one can be deprived of German citizenship. There is only one exception: When a corresponding law allows it and the person also has another nationality. The objective is to ensure that no one becomes stateless. There is, e.g. a law which says that a German citizen who has fought for ISIS can lose German citizenship- but only if he/she holds another (second) citizenship.
The article also states that German citizens cannot be extradited to another country. When another state wants to put a German citizen on trial, he/she has the right to remain in Germany. But here is an exception: if the court in question is an international court or within the European Union, the person can be handed over. The crucial prerequisite is that the legal process there is fair.