The “Page XX of YY” field tells you how many pages there are in the current topic (“YY”) and which page you are viewing at present (“XX”). 

Thus, e.g., “Page 1 of 4” indicates that there are four pages in the current topic, and you are presently viewing the first of these.

Note that you can jump directly to any page within the current topic by clicking this field and entering the number of the page you want to go to.