They're 1" long strips of 3M white reflective tape. Some people like a quarter of their wheel covered by it so it flashes as the wheel spins. I had extra time so I just covered the whole inner rim. Makes for two nice glowing rings of light in front of cars, it looks pretty cool.
As you can see though, the spoke reflectors don't work for squat. The reflective stuff though works good, under regular lighting without the flash the stickers are so filthy they're actually a dull gray. Which reminds me, I better wipe them down right now!