"Verso" is a book design term referring to the "left" or "back" side of a two-page spread of a left-to-right book. In a right-to-left book (like some books in Chinese, Japanese or Arabic), verso refers to the "right" side of the spread instead.

From Wikipedia:

Recto is the "right" or "front" side and verso is the "left" or "back" side when text is written or printed on a leaf of paper (folium) in a bound item such as a codex, book, broadsheet, or pamphlet.

By book publishing convention, the first page of a book, and sometimes of each section and chapter of a book, is a recto page, and hence all recto pages will have odd numbers and all verso pages will have even numbers.

Although many of us indie fiction authors deal mostly with ebooks, having terms like "verso" and "recto" in our vocabulary gives us more precise ways of articulating our needs if we ever dabble in book design. It's just the kind of arcane knowledge that could make life easier or one day get us out of a bind (lol).