This is a Lens Hood for a GoPro Hero 3.
A Lens Hood helps block glare from bright objects on the edges of the viewable frame. If you do aerial photography or fly FPV you might have noticed that "scanlines" can appear when flying fast and the propellers are slightly in frame. For FPV this is a mild annoyance, for aerial photographers this ruins the shot. This lens hood solves this issue.
I have tested it several times on my own setup (DJI Phantom w/Zenmuse Gimbal), and it works wonderfully. The only downside is that Jello is more likely to appear do to the increased surface area around the camera (basically the hood can catch wind and jostle the camera).
Installation is press fit, no glue required. It has a notch taken out of one side to allow fitment on a Hero 3 mounted in a ZenMuse Gimbal. Use Caution. I made the part as thin as possible to save weight. Due to this, it is somewhat fragile. As with any lens hood, the darker/matte material works best.
UPDATE: I HAVE DONE SOME MORE FLYING WITH THE HOOD INSTALLED. WHILE IT STILL PERFORMS IT'S JOB AT REDUCING GLARE, IT CAN MESS WITH SOME GIMBALS IN MID TO HIGH WIND CONDITIONS. KEEP THIS IN MIND WHEN USING IT.