KERS is a device that recovers the kinetic energy that is present in the waste heat which is created when the car breaks. It stores the energy and then transforms it into power which in turn can be used to as a booster upon acceleration.
KERS was introduced to develop an environmentally sustainable car as well as assisting drivers to overtake. Whilst there are limitations on the use of KERS, the boost button can help a driver pass the car in front and the leading driver can press his boost button to speed up and break free.
Source: Racecar Engineering