In an ideal op amp, there would be no current flowing into or out of the input pins. However, in a real op amp there are two sources of current. Bias current is the current that flows into or out of the input pins. Offset current is the difference between the current that flows to the inputs. When bias currents are very well matched, the offset current will be minimal. Bias current is normally much larger than the input offset current. Bias current is a problem for op amps because it can add errors when it flows through external resistance. For example, if the source impedance is 1MΏ, and the bias current of the op amp is 10nA, it will create a 10mV error. Or, if a signal is capacitively coupled to the op amp the bias current will charge the capacitor and stop working.