The Second Viennese School was a group of composers who made music that sounded different than the music other people were making. This group was made up of three famous composers named Arnold Schoenberg, Anton Webern, and Alban Berg.
These composers made music that had no set melody or rhythm like a lot of other music had. Instead, their music used something called atonality which means that there is no one note that sounds more important than any other note. The music sounded kind of like lots of different notes being played at the same time, kinda like a bunch of kids all shouting at the same time.
The Second Viennese School composers also used something called twelve-tone technique. That means they made sure to use all twelve notes of the musical scale equally, so no one note was used more than any other note. It's like they all shared the toys at the playground so nobody got left out.
These composers made music that was complex, meaning it had a lot of different things going on at the same time. They used different instruments and sounds to create their music, and sometimes they even used no instruments at all and just had people singing or shouting.
Even though some people didn't always like their music, the Second Viennese School composers were very famous and had a big impact on the music that came after them.