Personally, it seems this song is about women that may be extremely attractive, but really aren't inside, or really won't be for long. He compares them to fireflies, which pretend to be as brilliant and beautiful as stars -- but are just dim fakes that don't compare to the real deal (ie., a truly beautiful woman, inside and out). Reese could very well be talking about girls that go in and out of his life that seem like a possibility, but soon just flicker and fade out of his life.
Personally, it seems this song is about women that may be extremely attractive, but really aren't inside, or really won't be for long. He compares them to fireflies, which pretend to be as brilliant and beautiful as stars -- but are just dim fakes that don't compare to the real deal (ie., a truly beautiful woman, inside and out). Reese could very well be talking about girls that go in and out of his life that seem like a possibility, but soon just flicker and fade out of his life.