Harmonic current is the additional current in to main line current(principle) due to non linear load. Harmonic current is simply loss of power in the system. The value of currents depends on the harmonic order of currents. This harmonic order depends on the type of load.
harmonic current is a complex wave which consists not only of the fundamental frequency but contains harmonics. this inclusion of harmonics in the wave makes the current filled with harmonics.
An harmonic current is a current which frequency is a multiple of the power system frequency. For example, a 5th order harmonic current has a frequency of 250Hz for a 50Hz system.