Before reading this page read the one on LOGIC GATES.
The OR gate has two or more inputs and one output.
The output voltage goes high only when one or more input voltages are high.
In the switch diagram the lamp lights up when A OR B (or both) are operated.
In the truth table Z = 1 when A or B = 1.
The Boolean expression is A+B = Z which translated says, A or B high makes Z high.
The plus sign + translates as OR.