This is the current that flows in or out of the input pins. The input pins are the base pins of the transistor (or gate pins in the case of FET inputs). In order for the input transistor to operate, the base current (IB) must flow. This base current is the input bias current. In actual use, when a resistor with a high resistance (RIN) is inserted at the input, the input bias current is used to express the input signal error.
For example, if RIN=1MΩ and IB=100nA, there is an error of 0.1 V. Therefore, when the impedance of the signal source is large, it is necessary to select a product with a small input bias current. Note also that the difference between the bias current of the + and - input is the input offset current. Whether the bias current flows in or out differs depending on the input stage transistor.
Note Because the input bias current of a Pch JFET is equivalent to the gate leakage current, it has the feature that it is very small - in the order of pA units. However, if the temperature rises 10°C, the bias current doubles.