Arnold Schoenberg wasn’t the first composer to use elements of serialism, but he was the first to impose a structure this rigid: all twelve notes in the chromatic scale receive (more or less) equal attention, and key signatures as we know them are essentially irrelevant.
Not that I could explain it beyond that point. Fortunately, there is someone who can:
After seeing this for the first time, I mentioned on Twitter that I’d learned more from this half-hour of unconventional pedagogy than from a whole semester of theory. (And it is half an hour, so pour yourself a tall one and settle back.)