Before reading this page read the one on LOGIC GATES.
The NOR gate has two or more inputs and one output.
The output voltage goes low only when one or more input voltages are high.
In the switch diagram the lamp goes out when A OR B (or both) are operated. (A short circuit is placed across the lamp).
In the truth table Z = 0 when A or B = 1.
The Boolean expression is A+B = Z which translated says, A or B high makes Z low.
The plus sign, + translates as OR. Z (called Z bar) means Z is low.