Interest Rate Function

Question

I found some code online regarding a finance calculator function but I'm not sure what it does, I'm still new to swift so if someone could explain the purpose of the function I would appreciate it a lot

func calculateAnnualInterestRate() -> Double {
        let numberOfMonths = Double(self.NumberofPayments)
        var x = 1 + (((self.MonthlyPayment * numberOfMonths/self.loan) - 1) / 12)
        
        func F(_ x: Double) -> Double {
            let F = self.loan * x * pow(1 + x, numberOfMonths) / (pow(1 + x, numberOfMonths) - 1) - self.MonthlyPayment
            return F;
        }
        
        func F_Prime(_ x: Double) -> Double {
            let F_Prime = self.loan * pow(x + 1, numberOfMonths - 1) * (x * pow(x + 1, numberOfMonths) + pow(x + 1, numberOfMonths) - (numberOfMonths * x) - x - 1) / pow(pow(x + 1, numberOfMonths) - 1, 2)
            return F_Prime
        }
        
        while(abs(F(x)) > Double(0.000001)) {
            x = x - F(x) / F_Prime(x)
        }
        
        let I = Double(12 * x * 100)
        
        if I < 0 || I.isNaN || I.isInfinite {
            self.interest = 0.0;
            return self.interest
        } else {
            self.interest = I.roundTo2()
            return self.interest
        }
    }

I know that the function is necessary for the calculation of interest rate but I'm not sure what the F and F_Prime