Jonathan Alter explains in The Daily Beast:
Under Warren’s plan, inspired by Thomas Piketty’s influential 2014 book, Capital in the 21st Century, and drafted by Emmanuel Sanz and Gabriel Zucman of Berkeley, Americans with a net worth of more than $50 million would pay two percent a year on their wealth over that $50 million. American billionaires (who number around 680 right now) would pay three percent on their wealth over that $1 billion. In most years, two or three percent is far less than the appreciation of their assets. The vast majority of people paying the tax would still be getting richer every year.