What does the SSID have to do with a networks security concerning rainbow tables?

I have noticed in a few places people mention that if a popular SSID is used, it makes a network more vulnerable to rainbow table attacks. I saw this in one location and assumed they'd mistakenly used SSID in place of the word password, but I just saw it again in an answer on this website.

As far as I know the SSID is just the network name, and rainbow table attacks are concerned with password hashes, so what would using a common or popular network name have to do with making it easier for a password attack to be carried out on the network. Is the SSID used in any way, or is there some wierd correlation between SSIDs and common passwords?

Thanks!