Direct current (DC) does not have power factor because it does not alternate.
Power factor is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit.
Real power is the average of the instantaneous product of voltage and current and represents the capacity of the electricity for performing work.
Apparent power is the product of RMS current and voltage.
Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power, so more current flows in the circuit than would be required to transfer real power alone.
A power factor magnitude of less than one indicates that voltage and current are not in phase, reducing the average product of the two.