In a nutshell it's a method companies use to scam people. One example would be a company that poses as a group to make them selves look good by making others think they are popular etc.
It's also the act of covering a field with fake grass.
Astroturfing is the use of fake grassroots efforts that primarily focus on influencing public opinion and typically are funded by corporations and governmental entities to form opinions. Some studies suggest astroturfing can alter public viewpoints and create enough doubt to inhibit action.
Basically, when organizations put money into manipulating the public opinion by their side, it is called Astroturfing.
Astroterfing is the use of complex software in order to drown out the opinions of real users on Internet forums and replace their opinion with another, so to speak, necessary one. Today, this phenomenon is on the rise and, unfortunately, we can no longer be sure that what we read on the Web is written by real people.
Sponsors use astroturfing to promote sponsored (paid) content in a way that appears to come from a grassroots organization or an unconnected individual. (Astroturf = fake grass.) Because people trust neutral parties more than advertisements, some huge corporations use this tactic. It's widely seen as unethical.
Rather than relying on astroturfing, I encourage cultivating connections with dependable brand ambassadors.