Where does the white wire go on a light fitting?

In home wiring the white wire is neutral. Some light fixtures that are typically out of reach may not differentiate between hot (black) and neutral (white). In this case the black and white supply wires can arbitrarily connected to the two fixture black wires, one to each.

The NEC requires that the neutral be connected to the screw shell of a 'typical' light socket regardless of height. This allows that while changing out a lamp (called a bulb), if you inadvertently come into contact with the screw shell, you are not in danger.