You can prob find a better explanation online, but basically faking a “grass roots” movement using bots or a brigade of users. It’s called such since astroturf is fake grass.
The idea is to make it look like support for/aversion to something is organic and popular (grassroots) when it's entirely artificial (astroturf)
The key characteristic of astroturfing is that it attempts to influence public opinion by presenting a false picture of the current public opinion. It's both of those things together that make something astroturfing.