Tkinter employs a type of programming called Event-Driven which means that the program runs in a endless loop waiting for things (events) to happen. In order for the program to respond to these events it needs to know what to listen for.
The code above bind an s key press event to the say hello method.
def sayHello(event): print "hello"
Note how the method has an event parameter even though we have not set any parameters in the bind. This is because the event is automatically sent in the method call.