I’m sure you would find your answer if you search but the short answer is the driver is the electronic control circuitry between battery and LED to serve several purposes, namely the proper current and voltage to the LED. To understand why a driver is necessary is to understand that an LED is a diode with a forward voltage drop from anode to cathode when current is flowing and that voltage has to be reached otherwise no current flows and thus no light emitted. The typical LED has a forward voltage drop of around 3.0v before it begins to turn on and then once that happens, a very small increase in voltage takes it to its maximum limit with the risk of burning it out. Thus LED’s are best served via current control and not voltage control. The amount of current through an LED determines its brightness.
A good LED driver will basically be a small switching type of power supply that sources a fixed current depending on which mode you are in (low, med, or high), regardless of battery voltage (within reason of course). Since current is the determining factor in the circuit with it being the feedback mechanism in this power supply, the voltage will just go to whatever is needed to provide that current.
If the primary battery source voltage resides mostly below the 3.0v (single cell AA) then a boost type of driver is needed. If the battery voltage is above 3.0v (Lithium ion), then a buck type of driver is needed.
Summary:
- LED is a diode that is best suited via current control.
- Light output is proportional to current through the LED.
- You can’t just put a battery voltage on the LED unless you are precisely right around the Vfwd of the LED. You can do that with a “fine” adjust of a bench power supply
- Drivers boost or buck the battery voltage to the LED such that a constant regulated current flows.
Hope that helps in a few of words as possible without getting too technical.
Edit: I see that this was originally posted last year. Oh well, maybe if someone else comes across with the same question this can be useful.