Defense is all about reducing the odds of being hit. I know this seems pretty obvious, but it implies that an understanding of the attack mechanics are needed.
Every attack in City of Heros carries two 'tags' on it: Vector (How it's delivered) and Type (What it does) Fire Blast, for example, would be (ranged)(fire). A council grenade launcher might be (AoE)(Lethal). Super Reflexes is about positional defense; so it only cares about Vectors. In effect, you've reduced 'What do I need to defend against?" to three things: Ranged, Melee, and AoE.
Against an undefended target; enemies have a base hit chance of 50%. Which means they also have a 50% chance of missing; and generating misses is Super Reflexes' goal. For easy math, just add your Defense number to 50; and that's the likelyhood that an even-level, unbuffed mob will miss you with any given attack.
Except that, of course, no Defense is perfect and untouchable player characters would be broken. Because of this, there's an accuracy floor that will look at you and say "Nuh-uh." Regardless of Defenses, buffs, debuffs, the weather or the price of tea in china, there is always a 5% chance to hit. (5%= 1/20. So in D&D terms, a Natural 20 always hits). There is also a Streakbreaker that will take pity both on players and thug alike and force a hit. Because the computer's a cheating bastard. (Players also benefit from it, to be clear.)
So what does this mean for Super Reflexes? Well, if you can get your three Vector Defense numbers up to 45%, you will be as untouchable as the game mechanics allow for against all normal content. You can stand in front of Archvillians or giant monsters and they will miss time and again. Until they hit you and crush you like a bug because you've got no other mitigation. ^_^
Incidentally, the 'soft cap' for Defense rises to 59% in Incarnate content, just because stuff in there is higher level / buffed.
TL;DR: /SR is Peanut Butter & Jelly Sandwich simple. You only need three things. Bread (Melee Defense), Peanut Butter (Ranged Defense), and Jelly (AoE Defense) to have a perfectly enjoyable sandwich. Just don't eat too much.