Looks like this is a surface receiver. If you're RC car has a glow engine you would buy a 4.8V receiver battery.
If you have an electric powered car, you can use the battery that runs the motor to power the receiver.
Since the receiver only requires 4.8V and your car motor requires much more volts than that, the built in BEC in the receiver will convert the voltage from your motor battery to a lower voltage of 4.8 volts.
BEC stands for Battery Eliminator Circuit.
Below is a link to the manual for a Lynx controller which shows a diagrams of how the HP-2rnb can be wired up with and without the BEC.