What is Germany's 'Reichsbürger' movement?

The Reichsbürger are often radical and violent and do not recognize Germany as a legitimate state, and have increasingly 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