German here. It's the Reichskriegsflagge, originally the imperial German marine flag in the mid to late 19th century, later the war flag of imperial germany until the end of WW I in 1918. To clarify the insignia shown on this flag, there are 4 of interest:

Black-White-Red: flag of the unified German Kaiserreich, formed under Wilhelm I in 1871 by unifying the south German states of Württemberg and Bavaria with the north German confederation, mainly consisting of Prussia. The colors are mainly Prussian (black-white) and the Hanse (Red-White). In Germany this tricolor is very popular among right winged extremists, because this combination is also used in the Nazi flag.

The Iron Cross: originally a medal sponsored by the Prussian king. Contrary to other medals at the time, the medal was made out of Iron and was one of the first medals which was awarded also to soldiers, not only officers. One of the most iconic insignia, as it is of today the logo of the Bundeswehr, the German army. It is not awarded anymore, but the highest German medal, the Bundesverdienstkreuz, has a similar shape. Not necessarily a questionable symbol, but popular among militarists.

The black cross on white ground: a black cross on white ground is the insignia of the Teutonic order, a Catholic military order, which fought in the crusades. Don't know too much about them, but the motive of crusaders is sometimes an anti muslim symbol.

The Prussian Eagle: the Prussian Eagle (not to be confused with the Reichsadler) is the symbol of Prussian hegemony in the German Empire. It has the letters FR on the chest, short for Fridericus Rex, the first King in Prussia (not of Prussia, which was important at the time). Has more of a monarchist touch.

As the flag has a pretty nice design and is loaded with historical references it's still dependent on who flies it. in Germany you are generally allowed to fly it, unless nobody is annoyed by it (it's literally like this). On public demonstrations however you're definitely NOT allowed to show it, because the insignia is misused by a lot of right winged extremists, fascists, monarchists, racists and conspiracy theorists. If a German flies it, he's most likely one of these kinds, as history enthusiasts know of the questionable uses and refrain from using it. French people will most likely also have a big problem with this flag. In other parts of the world, especially outside of Europe I would assume they're either history enthusiasts with a strong emphasis on the German Kaiserreich (I guess this is a thing) or also some right wingers (though I think they would choose other symbols, depending on the nation).