What is SIX sigma?
The Sigma means the deviation in Greek language. The Deviation means how much variations exist in a set of data. For illustrate let's say in a software maintenance project out of 100 defects 68 defects are rectified to the mark and the remaining bounce back that means your bug fixing process is on "2 Sigma" level. I had described only from bug fixing perspective. But this can be applicable to any process organization.
Figure: - SIX Sigma Values