Ballasts supply power to your grow lamps.
The ballast provides the required voltage to start and operate the lamp. The ballast takes the available voltage from your electrical outlet and ramps it up to provide a high voltage pulse to start the lamp.
Once the lamp has started the ballast regulates the voltage to maintain a precise electrical arc inside the lamp as specified by the lamp manufacturer. If there are variations in line voltage supplied from your outlet, the ballast adjusts to help maintain the lamp.
As a lamp ages, it may require a higher voltage to operate correctly. The ballast will increase the needed voltage to maintain the electrical arc. If the aging lamp requires too much voltage, the ballast will shut down as a safety precaution. Your lamp may restart but it will continue to shut down because the voltage requirement is too high for the ballast. This is a typical sign that it is time to replace your lamp.