Does a diode allow electricity to flow in one direction?

Diodes come in so many different types and uses. Your statement does hold true to a certain point. They are highly valued for their diverse applications. The zener diode which is the rare exception because this semiconductor is usually reverse biased ( installed backwards) to function in the circuit. These were primarily designed to be voltage regulators available in a wide variety of voltages and types. In Engineering terms this operates in the avalanche region (reversed). All other types would run forward biased (in conduction). The anode is the positive end and the cathode negative.