A rear view mirror is a mirror, usually mounted on or just above the windscreen that allows you to see exactly what is happening behind you. Whilst it can come in many shapes and forms (see above), no matter which it is a key safety feature in any vehicle, even off road ATVs.
It can improve your driving experience and provide extra safety, which is why it is a good idea to upgrade the simple factory issued designs that most cars are supplied with as default.