It either refers to a method or a measurement.
As a method, it helps improve business operations by focusing on the effectiveness (quality or accuracy) of the output from a business process. It does this by finding ways to reduce variation in order to help reduce defects (instances that fall outside customer's requirements). The most common approach to applying it is the DMAIC methodology.
As a measurement, it is a statistical reference to having six standard deviations (sigmas) between the process mean and the customer requirements (i.e., LSL/USL). Having more sigmas in the process means there is very little variation and is generally referred to as having only 3.4 defects per million opportunities (DPMO) or being 99.9997% accurate.