DC Versus AC and the Diode
Direct current (DC) by definition flows in one direction. Alternating current (AC), on the other hand, periodically changes direction, or reverses polarity. It is indeed possible for a portion of a circuit that is normally DC to periodically change direction, or reverse polarity like an AC circuit.
Examples can include some 555 timer circuits and circuits that drive DC motors. Some electronic components can be sensitive to reverse polarity and can be damaged by sending electricity through them the wrong way.
To protect these components, you can install a diode in the circuit. You can build a simple circuit to demonstrate how a diode can block the flow of current.
Please see my article at Science 2.0 for a more complete discussion and build instructions.