*puts lit cap on*
it is about plato's republic.
also referred to as "the cave parable"
people (that's you and me) sit in the back of a cave in the dark, adn when they realize that the darkness is not the truth, some seek to find the truth, they then venture up to the mouth of the cave. they are mostly mislead by a reflection of light that is bounced off the cave wall onto them. many more are lost staring at this reflection of the light (light = truth). so just looking at the reflection of the light is only to accept a half-truth. others press on, they proceed toward the mouth of the cave. however, the light, the pure light, burns peoples' eyes, so many turn back because they were more comfortable staring at the reflection or at the blackness of the very back of the cave.
upon reching the light, accepting the light, and standing in the light, everyone goes back into the cave to start the journey over and over again.
the insect that is referred to is the gadfly. it 'burns' and 'scratches' and 'stings' because it entices people toward the light, asking why and other such questions.
"afraid of what you'll find
so you step back inside"
clearly that's about being fearful of the truth, so not venturing outside of the cave.
"not the only one, who'd rather go blind"
rather than sitting in the cave in his own reality, bono is saying how he would rather get that one bit of truth than die knowing nothing. which is, unfortunately, not how most people are.
in conclusion, i hope that helped you out some.