If you are working through the material in "natural order", this usually takes you to the same place as "back' on your browser. It is useful if you arrive at some place by "browsing" and want to determine the context in which the material you find is presented.
When there is no clearly defined immediatly higher organizational level, as in the case of graphics referred to in several contexts, this button is absent. In the present document, the button is inoperative.