Hi
The rail is just another word for output line. Some PSUs have the 12v output as a single line and some have it split into anything up to about eight lines. The spec often looks like this 12v1, 12v2, 12v3, 12v4 etc that would be an example of a four rail output. Quite often iot will have the maximum load after each rail number so it woulkd look like this 12v1 - 18A. This means that the most power available for anything connected to this rail(cable loom) would be 18A or 490W.
As a general rule it is better to have a single 12v outlet rail if possible.
Hope this helps.