They're defined above.
fft_sine is the result of the FFT, either as complex number, or the modulus of that complex number.
N is the number of bins (or the number of samples).
mean_window is the average value of the window function you're using.
V_ref is the ADC reference voltage, it depends on what Arduino you're using and how it's configured.
ADC_factor = 2^10 = 1024 if you're using the internal 10-bit ADC.