it separates nodes in a path
you generally want full paths like this in the event you have two nodes with an identical name
for example if you have to following nodes:
|group1|pCube1
|group2|pCube1
using "pCube1" in a function will raise an error because MEL doesn't know which of the two nodes you mean
the ls command has the -l flag to get full paths, the listRelatives command has the -f flag to do this, making a habit of this will save you a lot of headaches
Cheers,
-Trevor