Astroturfing is a term for formal public relations campaigns in politics and advertising that seek to create the impression of being spontaneous, grassroots behavior. Hence the reference to AstroTurf (artificial grass) is a metaphor to indicate fake grassroots support.

The goal of such a campaign is to disguise the agenda of a client as an independent public reaction to some political entity—a politician, political group, product, service or event. Astroturfers attempt to orchestrate the actions of apparently diverse and geographically distributed individuals, by both overt ("outreach," "awareness," etc.) and covert (disinformation) means. Astroturfing may be undertaken by anything from an individual pushing their own personal agenda through to highly organized professional groups with financial backing from large corporations, non-profits, or activist organizations.

from wikipedia