What does the manual mode do on your digital camera?

Manual Mode allows the photographer to set the Aperture, Shutter Speed and ISO. The camera will not change these settings while it remains in Manual Mode.

Automatic Mode tends to set proper exposures in most situations. Where Automatic Mode typically struggles is in very high contrast lighting. A dance recital, for example, might have brightly lit performers on very dark (unlit) backgrounds. Automatic Mode will often try to split the difference between the brightly lit performer and the dark background, resulting in an overexposed performer.

In this situation Manual Mode would allow the photographer to "dial-in" the correct exposure and keep it locked for the duration of the performance.