Basically, it's the same as the Orbital Fleet Manager, except that it allows you to defend with all the ships in orbit, NOT just as many as you can fit in a normal fleet.
Basically, what this means is that under normal conditions, when an enemy attacks one of your planets, you defend with one ship at a time. If the enemy has a decent fleet and that one ship isn't awesome, it's going to be toast. With an orbital fleet manager, you can defend with a fleet, rather than a single ship. This fleet will still be limited by your logistics skill, so early on it will be 2-4 ships, though unless you're behind in tech, that will give them an approximately equal footing. With the Hyperion version, ALL ships in orbit of the planet form a fleet to defend the planet, disbanding the fleet after the attack. Given that you can't fit 10 shiips in a fleet until towards the end of the game, this can make a planet quite painful to try to invade.