The basic idea is to get every major city in the world involved in this organised event! Flashmobs like this are getting more and more common, and I think its a great way to let loose amidst our often stressful working weeks!
The first flashmob apparently took place in Macy's, New York, in 2003. The Oxford Dictionary describes the flashmob as an 'unusual and pointless act' - basically separating it from more serious gatherings like protests and so on. But that is the point - its not meant to mean anything serious, its all about having fun and now the world is going crazy with silent discos and pillow fights!
Some tips - don't hit people too hard with your pillows! They might be made of feathers but can still pack a punch (I would know, having been on the receiving end!). It might be a difficult rule to remember once you get into the swing of it, but still an important safety rule!