This is what the documentation says:
If the first responder [to an event or action message] cannot handle an event or action message, it forwards it to the “next responder” in a linked series called the responder chain. The responder chain allows responder objects to transfer responsibility for handling an event or action message to other objects in the application.
If an object in the responder chain cannot handle the event or action, it resends the message to the next responder in the chain. The message travels up the chain, toward higher-level objects, until it is handled. If it isn't handled, the application discards it.
Okay, what is the next responder?
Is it the parent view? The view behind it? How does iOS decide what is the first responder and second responder?
The First Responder is a particular concept in Cocoa. The only time iOS decides to set the First Responder is when a text field gets focused. At all other times, you must explicitly control which object you want to be the first responder (see -canBecomeFirstResponder, -becomeFirstResponder).
There is no such thing as a second responder.
All responders have a NextResponder (which can be nil). This means that starting from any responder there may be (but may not be) an arbitrarily length chain of responders (responder → nextResponder → nextResponder → etc) along which events are passed until they are handled.
There is a default chain which can be view → superview → superview but might also include UIViewControllers, UIWindows, UIWindowControllers, UIApplication and more, so it heavily depends on your object hierarchy (not just your view hierarchy - so no, you can't say nextResponder is always the parent view). On OSX 10.6 the default chain is even different for different types of events and actions and can even include your app delegate, which may or may not be a responder, i'm not sure if this is the case in iOS tho.
The default chain is only the default, though. After you have designated an object as the First Responder, it is up to you to insert, remove, and append items to its responder chain to achieve your desired aim.
The responder chain is quite important and complicated, you should take time to read the Apple docs about it.