I'm going to use some D&D terms, because they might be familiar. Parry defense is defense against melee attacks. So add 10 to your Parry to get your "Armor Class" for melee attacks. It determines how hard it is for enemies to hit you in melee. 10+Dodge is also how hard it is to hit you with ranged attacks. Fortitude and Will are usually actively rolled.
So, for example:
Hibiki wants to punch Chris, so she gets in close and rolls her attack. She rolls a 5 on her d20, and adds 10 for her accuracy (never mind how she got it). This doesn't beat Chris' passive parry defense/AC of 20, so she misses.
An example of saving against an effect:
Early in A's, Nanoha's magical powers are drained by Shamal. In M&M terms, Nanoha failed her Fortitude save to resist the Drain effect, which would have a DC of 10+Power ranks.
Damaging effects have a DC of 15+ranks, so it's slightly easier to get hurt than other stuff. It's also less important to have your other saves as high as Toughness.