A DI box serves two purposes normally. The first ia to split the signal so that you can send it to both your amp and the main mixing desk of your PA or Recording system. The second is to match the impedance. Most guitars etx have a high impedance unbalance output, where as most mixing desks have a low impedance balanced input. DI boxes come in to basic types: Passive, which use transformers, and Active which use (essentially) an amplifier. Active ones require a power source: either a battery or they can use the phantom power output from the mixing desk (the same one as used by condenser mikes)