Search for it on the page, and you'll find this bit of explanation and a whole lot more...
"Packages that are not expected to change can be specifically configured with a volatility (V) value in the jdepend.properties file. (...) For example, a package like java.lang is generally not volatile. That is, for all practical purposes this package is maximally stable. Creating dependencies on it is not cause for concern. Thus, if you include this package in analysis, it's best to set its V=0."