The rating system is known to be similar to YouTube's own rating system.
Ratings are monitored by the ROBLOX Staff. In any game in particular that is massively down-voted, the staff will investigate the activity to make sure it's not malicious in nature (such as a single player creating alts to dislike a game).
The Game Ratings feature is planned to be integrated into the Games page (but not as a sort). Games that are overwhelmed with "thumbs down" votes would be pushed down the game page, and eventually off the games page entirely, even if it is being visited frequently. Additionally, there are plans to expand this feature to all assets on the site.