This is a term being thrown around a lot these last few weeks. It means building up phony grassroots support for your cause.
The Dems have been accused of planting questioners in front rows of these town hall meetings. If you are from a group that supports the Obama plan, you get front row seating and side door entry. Everyone else has to stand in line and gets rear room seating or standing room. This way the newspaper reporters and news cameras covering the speaker are closest the the planted astroturfers.
The speaker plays the game and pretends these people are getting chosen at random. Barry O seems to be a big player in rigging the questions through astroturfers.
Here's a segment from Texas. This lady claims to be a doctor. She's not. She's a long time Obama supporter lying in the name of astroturfing.