It's when people (or bots) acting in bad faith make a lot of noise to make it seem like there is a strong support for some (usually bad) position. It' a play on

It's when people (or bots) acting in bad faith make a lot of noise to make it seem like there is a strong support for some (usually bad) position. It' a play on the term "grassroots". Fake grassroots=astroturfing