G'day Mate,
Welcome to this great hobby.
The usual voltage for Recievers & servos is 4.8Volts, ie 4 cell battery packs, & this is what you get when you buy a radio system, but you can safely use 6Volts, ie 5 cell battery packs, that is why they list 2 voltages, & speed/torque ranges for servos. I won't go into detail about the pro's & cons of the different voltages.
On most radios these days, they have a switch on the Elevator, & Aileron axis, & some on rudder too, which allows the pilot to set different amounts of servo travel, at full stick deflection.
On the lower deflection, or RATE, the plane's controls will be less sensitive to control inputs, & thus make it easier to control, if more control response is needed, then a flick of the switch, & instantly you have more, or less control surface deflection, depending on which way you move the switch.