On a modern bicycle, the rear gears (called a cassette) are separated from the spokes of the rear wheel by a plastic disc. The purpose is a preventative measure if the chain slips off into the wheel, the spokes are not destroyed; which keeps the rider safer from injury. However, if the limits on the derailleur are adjusted correctly, this should never be an issue. Typically, the plastic discs are removed from a newly purchased bike, else the cyclist will be considered a newbie or noob by colleagues.
Mr. Spandex: "You haven't removed your noob guard yet?"
New Biker: "Isn't it there for a purpose?"
Mr. Spandex: "It's just extra weight!"