Stirring can add oxygen into the wort (now beer as it has alcohol present) and skunk it. However, oxygen also helps the yeasties out, as it encourages them. It's a good thing when you start fermentation, not after it's been going for 2 weeks.
When most people here say "Stir" on a brew, it is done by swirling the fermentation vessel to get the yeast cake back in suspension, or if additives have been added they want back in suspension. They are then allowed to settle back out before racking. During this "Stirring" process, the fermentation vessel remains closed so as to not allow oxygen into the beer or wine/mead.