Null Meter

I'm assuming that you are looking at a GCP (Generator Control Panel) with a Synchroscope and a Voltage Null Meter for synchronizing with other machines or the grid. Prior to synchronization the null meter is there to enable you to see that the generator terminal voltage matches the magnitude of the voltage on the grid side of the generator breaker. The null point is straight up, meaning that the magnitudes are matched, pointer to the left, generator voltage too low, pointer to the right, too high relative to the grid. You use the Voltage Setting Control on the AVR to match the voltages.

But wait, there's more. Just matching the magnitudes is a necessary, but not sufficient, condition, you also have to match the frequency (speed) of the grid, and its phase angle. That's where the synchroscope, another type of null meter, is used. Rotating to the left (counterclockwise) means the generator is running slower than the grid, rotating to the right means it's running faster, not rotating and not vertical means the speeds are matched but the phase angles are not, you have to incrementally adjust the generator speed control to have the pointer straight up.

At this point you have satisfied all three requirements for synchronizing; voltage magnitudes matched, frequencies matched, and phase angles matched; now you can safely close the generator breaker. If all three of these parameters are not closely controlled when the breaker is closed then bad things can happen very quickly.