"grass-roots" means: coming from the bottom of society, or having support from ordinary people
Example:
"TOKYO - Japan's ruling party picked grass-roots populist Naoto Kan as its new chief Friday, paving his way to replace political blueblood Yukio Hatoyama as prime minister while the party struggles to reclaim public support ahead of July elections.
"...Unlike recent prime ministers, he was not born into an elite political family. Several past prime ministers, including Hatoyama, had fathers or grandfathers who were also prime ministers.
"I grew up in a typical Japanese salaryman's family," Kan said Thursday. "I've had no special connections. If I can take on a major role starting from such an ordinary background, that would be a very positive thing for Japanese politics."
