The switch controls the brake lights when the brakes are applied. The signal from the switch is also used to disable the cruise control system when braking.
To remove the switch, disconnect the electrical connector and loosen the locknut. Then remove the switch. When installing the switch, adjust it with a locknut so that it operates during the first 13 mm of pedal travel. If the switch is installed with no slack or free play, a hard hit while driving can disable the cruise control and/or cause the rear brake lights to flash.