Android: Overdraw, What is it and Why Should You Care?

I recently read this blog post,  Android Performance Case Study by Romain Guy, on profiling an application that he used regularly to determine why there were occasional hiccups.  I used one of the tools he recommended, Debug GPU Overdraw, to see how the PostTV Application was behaving. With one minor tweak I was able to reduce the overdraw by 1x.  So what does overdraw mean anyway?