Resistor example 1D - DC solve explanation

This page explains the following code snipped from the 1D Resistor example:

DC solve / plot I-V as output

set Win [CreateGraphWindow]
set bias 0.0
for {set bias 0.0} {$bias < 1.01} {set bias [expr $bias+0.1]} {
	contact name=VSS supply = $bias
	device
	set cur [expr abs([contact name=VSS sol=Elec flux] - [contact name=VSS sol=Hole flux])]
	AddtoLine $Win I $bias $cur
}