The callbacks here are the methods defined by the OnPageChangeListener: Callback_(computer_programming). They're "called back" from code other than where they are defined. You may not have heard the term, but anyone using event-driven APIs (like GUIs, whether Android or AWT/Swing) will be familiar with the concept.
Um, I think you missed pretty much all the points I've been trying to make in all my replies. More directly: Why are you listening for events in the ViewPager if you're actually interested in events concerning the images (or whatever View they are part of, which you haven't said so far)? If you are interested in "click" or "touch" events, why are you listening for "page scroll" or "page select" events?
I'm sure you can figure it out on your own; that'll be much more useful as a learning experience for you. Look up the javadocs of the View holding the images to learn what listeners you can attach to it. Don't forget to check the superclass(es) of it as well. Which listeners are those? Which of those suggest themselves if you want to handle clicking on it?
A hint: listeners are generally attached via methods called "setOn..."