Professor accused of "self-plagiarizing"

What's self-plagiarism?  Is like re-cycling or re-purposing?  How seriously should anyone take it?  If the case of University of Toronto researcher Stephen Mathews is to be believed, it's a "severe abuse of the scientific publishing system." Stephens and a couple of colleagues are accused of re-using stuff they wrote in 2005 documents as part of current published work. The scientific journal Neuroscience and Biobehavioral Reviews has published a prominent retraction of certain of Stephens work. Vancouver Sun