A light ballast controls the current running through a fluorescent light bulb. These elements are necessary to limit the electricity running through the light. Without them, a fluorescent lamp would quickly draw in too much energy, resulting in an overheating burnout.
Light ballasts provide one quick burst of high voltage to illuminate fluorescent bulbs in the first place by connecting the electrodes with an arc of current. After that, they strictly limit the flow to preserve the lamp.
Some can also heat the electrodes with low voltage to increase lifetime operation of the bulb. Because these operations are so precise, ballasts tend to be built specifically for the lights they power, as opposed to being standardized for use with many different models.