Hi! Monster armor class is like character armor class: the lower it is, the harder the monster is to hit (since to hit, you need to roll 1d20 and get a result equal to or lower than your fighter level plus the opponent's armor class, plus any other modifiers).
Regarding monster saving throws: yes, they have a common roll for everything. I realize now that I could have explained it better. Maybe after the jam, I'll correct the text to make it more understandable.