The document discusses the observer design pattern, which defines a one-to-many dependency between objects so that when one object changes state, all its dependent objects are notified automatically. It describes observers registering with providers to receive notifications of predefined events or state changes. When such an event occurs, the provider notifies all observers by calling one of their methods and can provide current state information. Real-life examples of auctions are provided, where bidders observe changes in bid price. Implementation in .NET using IObservable and IObserver interfaces is also summarized.