This is a Hobbywing 1S DC/DC Booster.
This module is an external dc-dc booster; it draws DC voltage from 1S li Po battery, boosts it to 6V for your receiver & Servo, and keeps providing the stable output current of 3amp or above. As the beck voltage of 1S li Po, which used by 1/12th scale race cars, usually goes below 4.2V and causes the receiver and Servo to work abnormally, so a separate battery is needed to power the receiver. This booster can work as a receiver battery and save you the trouble of battery charging and maintenance.