What is the bubbling concept?

If you are asking about the term, I guess it is an analogy to an event 'bubbling' up to the top, like an air bubble does in liquid.

If you are asking what event bubbling is it is an event that is caught by one object that will refire it to any other objects that are listening to it.

To quote a good article here

... a technique called event bubbling that allows a child control to propagate events up its containment hierarchy. Event bubbling enables events to be raised from a more convenient location in the controls hierarchy and allows event handlers to be attached to the original control as well as to the control that exposes the bubbled event.