Well from my understanding a "rail" is talking about the voltages that a PSU supplies. That is, if you have a 3.3V rail it means the PSU delivers 3.3v on some of the wires. If you have a 5v rail it means the PSU suppliers 5v on some of the wires and a 12v rail means the PSU puts out 12v across the other wires.
If they say the PSU has more than one "rail" for the 12v what they mean is that the 12v wires don't "share" the 12v connection, instead they all have their own set of wires and connectors and its a better way of delivering full 12v to power things such as the graphics card, cooling systems etc.
Having more than one 12v rail is a more stable way of delivering the power, to put it basically.
If your using pretty new hardware such as Core Duo type motherboard and CPU, having more than 1 12v rail