Wikipedia
Push-to-pass is a mechanism on a racing car which provides the driver with the ability to increase the car's power for short periods of time, usually via a button on the steering wheel. The system is designed to make overtaking easier and hence make the sport more exciting to watch. At the present time all racing associations except drag racing ban nitrous oxide for use in boosting engines.
Several racing series have implemented push-to-pass systems, including the former Champ Car series, the A1GP series (who refer to their system as PowerBoost) and the Indy Racing League (who refer to the system as an overtake button).
There is no requirement that the system be used only when attempting to overtake another car. For example, a driver may choose to operate the system in order to defend a position. The amount of extra power, the amount of time it's available and the number of boosts available during the race varies from series to series.
In Formula One, the Kinetic energy recovery system functions in a similar manner; however, it relies on regenerative braking to store energy for later use, rather than simply increasing the engine's power, as is the case in some other systems. The Drag reduction system, by contrast, increases acceleration and top speed by moving an element of the rear wing to reduce drag rather than by boosting power; its usage in races is more restricted.