A hiatus is an interruption. The word is used to describe a break in the continuity of a work, a series, or some action. When a TV network says that your favorite TV series is "on hiatus," there is the prospect that at some future time the series will resume and its fans will be pleased. Which, of course, hardly ever happens.
In optics, the hiatus (Fig. H1.1) is the distance between two lens planes, the principal planes. (They will be discussed in Chapter 4.) When an axial ray enters a lens and hits the front principal plane PF, it jumps across the hiatus at the same height to the back principal PB and exits the lens.
So, this section of the text is an interruption. It is not directly related to what has just been discussed and what follows it. However, for some readers the information provided here will give you a better sense of the development of optical design. It will also provide you the tools and ideas needed to design and perfect an optical design. And it will help you appreciate the work that others have done in this field.