What are ballasts and why do i need them?

If you look closely at the bulb you will see no it has no filament. Instead it has gases that need to be ignited with electricity. The bulb has a gap where the gases are contained, electricity needs to jump over the gap to ignite the gases. To do that jump it needs a big boost, that is where the ballast comes in. After the jump the ballast lowers the power to the minimum level needed to keep the arc going.
You will notice when you turn on your HID’s it produces a really bright light then dims down and later warms up to be slightly brighter.