How to understand the core code in FFT?

[Einsturing]

Hello! I can't understand the calculation method in FFT.
o1_real = x.real * w1[0] - x.imag * w1[1] + b1[0]
o1_imag = x.imag * w1[0] + x.real * w1[1] + b1[1]
o2_real = o1_real * w2[0] - o1_imag * w2[1] + b2[0]
o2_imag = o1_imag * w2[0] + o2_real * w2[1] + b2[1]
Why should the real be '-', '+', '+', the imag be '+', '+', '+'? What role does this calculation combination play in it? Can I change the first '-' to '+' or vice versa?
And what role does 'kept_modes' play in it? Dropout?