Input bias current (IB) is the current flowing through an op-amp's inputs. Due to op-amp biasing requirements and normal operation leakage, a very small amount of current (pA or nA range, depending on the technology) is flowing through its inputs. This can cause issues when large value resistors or sources with higher output impedances are connected at the op-amp inputs. This leads to relevant voltage drops at the op-amp input and therefore errors.