For those of you who don't know, it's the rate of compression of your music. The standard is 128kbps.
Most of my music on my playlist are at 56kbps. Takes up 30-40% of the original space with little sound quality loss from 128kbps.
Since iTunes has a very enhanced media player(it primarily uses Quicktime Pro), the sound quality doesn't sound too noticable at 56kbps from 128kbps. So yeah.