Six-sigma is the appliance of the "Laws of Physics" to the "problem" of Engineering (of ALL disciplines) to obtain advantage.
EVERYTHING follows a standard distribution...by accepting that, and applying statistical tools to utilise the standard deviation (or sigma), the processes can be "manipulated" such that the likelihood of failure becomes infinitesimally small (along with the costs of any failure), and thus the quality of the highest possible standard.
Taking this to the sixth-level of sigma means a defect-free run of 99.99966% success in each individual opportunity or step.