Looks like a derailleur guard
It mounts to the rear drop out on the derailleur side.
It protects the derailleur from some impact damage.
The comments seem to be running strongly against the need for this device.
Many experienced riders see this device as useless, and for their use case this is true.
One possible use case: If I had a child and this was his/her first derailleur bicycle I would see this device as useful to prevent the occasional bike falling over or mild crash from affecting the derailleur.
Yes, the derailleur is inexpensive and the device is heavy - but the kid is just learning to ride and it does offer a little protection.
As a side note - when assembling a bike from the box
- Adjust the front hub before installing the wheel
- Pull the rear wheel and check / adjust the rear hub
- Slip the chain off the front chain ring (you can shift the chain into the smallest chainring and then slip it off toward the seat tube so that it sits on the bottom bracket) and check crank bearing adjustment
- Check headset bearing adjustment.
All bearings should spin smoothly with no play.
Many new bikes have bearings that are silly tight. A quick check and correct adjustment will make your bike ride better and last longer.