Use of resistors is the last resort if anti-flicker canbus decoders do not work and your vehicle's lights are still shutting off, hyperflashing, or generating an error code on the dash. Load resistors are generally used after an LED or HID upgrade to increase the power draw on an electrical system in order to simulate use of a halogen bulb. Modern vehicle's canbus electrical system keeps checking for burned out bulbs and by replacing halogen bulbs with low energy draw LED or HID bulbs, the canbus system will perceive the low load as the bulb being burned out, and will report it to the driver.
Most of the resistor packs for headlight upgrades are plug-n-play, except for where they need to be added to signal, brake, or reverse lights. In those cases, the resistors must be connected between the + and - wires leading to the bulb that was upgraded to LED.
Caution: Resistors get very hot, so they should always be mounted on metal surface where the heat will dissipate fast and withut affecting any plastic parts around it.