Alexa will read the cards, asking "Side 1" and you will have to respond to the question. The correct answer is on "Side 2".
By saying "Reverse", Alexa will start using the "Side 2" to ask the question, the answer now coming from "Side 1".
There also is the possibility to say "Random" to let Alexa decide what side will be used for questioning.
Note that some card sets do not allow the use of the commands "Reverse" or "Random"... For instance, the card asking "What is 5+3 ?" cannot be used the other way as there would be an infinite possibility of responses.