The roman numerals at the bottom tell you which level you'll cast this spell at. In this case you'd use a first ( I ) level spell slot to cast Mage Armour. A second level spell would be depicted as ( II ) and so on... You'll notice this once you unlock 2nd level spell slots, because then you can cast Mage Armour at higher levels in case you run out of 1st level spell slots.
Took a moment during my first session to figure out the differences as well.
