What is the Younger Dryas?

I stumbled upon THIS ARTICLE and was impressed by its clarity in describing the Younger Dryas. I knew that the Younger Dryas existed but I must have missed the lecture when it was defined. And I was vaguely aware that there was some controversies associated with it.

The Skeptoid web site specialises in debunking the fake science which proliferates on the internet and is well worth supporting - and you get clear descriptions of the Younger Dryas!