For example, look at what happens when we do nothing but change the text size of an ePUB, shown here in Adobe ePUB reader, of Mark Twain's beloved, "The Prince and the Pauper."  In the example on the left-hand-side, the text is set at the default medium size, and we've highlighted some text on the same "page" as the image in purple.  In the first image, on the left, you see that the text occurs on the same "page" as the image.  When we resize the font, however, by using the lower-case "a's" at the top of the reading pane, merely one size larger, you'll see that the very same highlighted text "moves" to the next page (screen), and is no longer visible on the same screen (page) as the illustration. This is because it "reflows," using larger letters, fewer characters and words per line, and the entire book is re-rendered, from beginning to end. In this example, the highlighted text moves from being on the same "page" (screen) as the illustration, to the next "page" (screen), because it's reflowing to accommodate the larger font size.  

This is what we mean when we say that text “reflows" and that it is very difficult to try to control what a user sees at any given time.  EBooks are not like PDFs or print, and it will be very helpful for you to understand this early in the process so you can learn not to worry about things that cannot be controlled.