What is Germany's 'Reichsbürger' movement?

They are radical and violent and do not recognize Germany's democratic state. In recent years, the Reichsbürger have drawn the attention of authorities. Who are they, and what kind of danger do they pose?

source https://www.dw.com/en/what-is-germany-s-reichsbürger-movement/a-36094740?maca=en-rss-en-ger-1023-rdf