In 2022, it was ruled that the BfV may classify and monitor the entire party as a suspected right-wing extremist group. A corresponding lawsuit by the AfD was dismissed because "there were sufficient factual indications of anti-constitutional efforts within the AfD".[36] On 26 April 2023, the BfV, after four years of investigations into the Young Alternative for Germany, categorized that group as a confirmed extremist organization. This allowed the chief of the BfV Thomas Haldenwang to place the youth wing under even more intensive surveillance than the tapping of phone and the use of undercover agents that had been the case until then.[45][46]

https://en.m.wikipedia.org/wiki/Alternative_for_Germany

Just summarizing here. They are right wing extremists that have taken over old Nazi talking points and are gaining popularity with anti immigration, anti climate change and populist propaganda.