It comes form Polish "po".
In Polish: Ona zwykle czyta (po) dwie książki miesięcznie.
In Esperanto: Ŝi kutime legas (po) du libroj monate.
But it seems that without "po" the sentance would also be understandable. Besides, you can also say: "Ŝi kutime legas du librojn ĉiumonate"
Po can be a tricky word. In the time since I commented in this thread, I’ve rethought a few details. I’m compiling my thoughts in this thread here. If you have questions about po, you might want to check it out.