There’s a definition on wikipedia. It’s much much wider than my tiny vision of this business management strategy.
But basically this strategy says : Fix the source of every problem and not each problem. In a factory, it means that have to change the production process to reduce the probability of making defective pieces instead of fixing pieces faster. That means you have to spend time on fixing the problem. So this stategy also means : Spend time to save time.
How that applies to me or you ? Everywhere. For me it can be seen as the creation of automated processes to replace some humanely driven processes. If you’ve already done a task twice and it can be optimized / automatized, then do it.
I’ve seen a lot of Sharepoint projects that would benefit of some “six sigma” thoughts. They often end up with a huge installation manual instead of optimizing the Sharepoint deployment package of each project.