"Peek" is the mechanism by which you access Hub. If you've ever used the Facebook app on Apple's iOS or Google's Android, you have a sense of how Peek works. Hub exists as a layer under the homescreen, and Peek allows you to slide the homescreen away like a curtain, revealing the real-time updates provided by Hub.
The beauty of it is that you never have to exit whatever app you're in completely. You can just check and see what's new, and then go back to business.
CNNMoney's hands-on impressions of the phone BlackBerry is counting on to be its savior.