So, you won't just review flashcards randomly. Instead, the app will stagger the questions for you so that you don't have time to forget any of the cards.
There is an option that allows you to add flashcards, specifying how important it is for you to learn each one, which will make it so that the cards appear more or less frequently. You can also indicate how well or poorly you do with each one.
Using all of this information, Mnemosyne is able to show you flashcards according to what you need. It will display them the way that you have set it to do so: front to back or randomly.