Analysing an op-amp circuit and I do not understand a specific resistor value
Input bias currents produced by op-amps (no matter how small) can cause error voltages when there are resistors attached to those inputs. So if the bias current is the same in or out of both inputs and the resistances feeding those inputs are the same then those errors are cancelled.

So, the equivalent resistance of the inverting input is the parallel sum of feedback and input resistor i.e. the formula in your question. If this parallel sum equals the resistance in the non-inverting input then the effects of bias currents can cancel out. I say can and this is usually but not always the case.